Price Theory in Action: How Demand and Supply Shape Prices

Petrol prices change almost weekly. Onion prices spike before every festival season. Curd gets pricier in summer, and mobile data has become dramatically cheaper over the last decade. Behind every one of these everyday observations sits the same economic machinery: demand and supply pulling against each other until a price settles. Price theory is simply the formal study of how that settling happens, and it explains far more about your grocery bill than most people realise.

What price theory really explains 🔗

Price theory looks at how the price of a good or service gets determined in a market, and how that price changes when conditions change. It is not about what a product “should” cost based on how much effort went into making it. It is about what buyers are willing to pay and what sellers are willing to accept, and how those two positions meet at a single point called the equilibrium price.

This idea sounds obvious today, but it took centuries of debate among economists to arrive at the demand-and-supply framework taught in every introductory economics course now, including in a B.Com curriculum.

The economists who shaped price theory 🔗

Three names come up repeatedly when price determination is discussed: Adam Smith, Karl Marx, and Alfred Marshall. Each approached the question of “what decides a price” from a different angle.

Adam Smith and the classical starting point 🔗

Adam Smith argued that the labour required to produce a good was the common thread running through the value of everything. He distinguished between a good’s “natural price,” which covers the cost of production, and its “market price,” which is what it actually sells for on a given day, often above or below the natural price depending on demand. Smith’s classical successors largely followed this distinction between market and natural prices, even when they disagreed on exactly how natural prices were set. Smith and his contemporaries also ran into a puzzle they could never fully resolve: water, essential for survival, is nearly free, while diamonds, which serve no practical purpose, sell for enormous sums. This became known as the water-diamond paradox, and it exposed a gap in any theory of price based on labour or usefulness alone.

Karl Marx and the labour theory of value 🔗

Marx built on Smith’s foundation and pushed it further. He argued that the value of a commodity comes from the total amount of socially necessary labour required to produce it, meaning the labour time an average worker would need under normal conditions with standard tools. Marx used this idea to explain where profit comes from in a capitalist system, arguing that workers create more value than they are paid in wages, and that the difference becomes surplus value captured by producers. His framework was less about predicting day-to-day price movements and more about explaining the distribution of income between labour and capital.

Alfred Marshall’s synthesis 🔗

Marshall is the economist whose ideas most directly shape the demand-supply diagrams used in classrooms today. He rejected the idea that price comes from cost alone (the supply side) or from usefulness alone (the demand side). Instead, he compared the two forces to the blades of a pair of scissors: neither blade cuts on its own, and it is the interaction of both that produces the result. Marshall also broke down markets by time horizon, describing temporary, short-period, and long-period equilibrium prices, since demand can react to a price change almost instantly while supply, especially for manufactured goods or agricultural produce, often needs weeks, months, or years to catch up.

Reading the demand curve 🔗

The demand curve plots how much of a good buyers want to purchase at each possible price. It slopes downward because, all else being equal, people buy more of something when it gets cheaper and less when it gets expensive.

Movement along the curve vs a shift of the curve 🔗

This distinction trips up most students at first, so it is worth being precise about it. When only the price of the good itself changes, the quantity demanded moves along the same curve. When something other than price changes, such as consumer income, tastes, population, or the price of a related good, the entire curve shifts to a new position. Economists call these non-price factors demand shifters, and an increase in income is one of the most common ones.

Example: rising income and the price of fresh curd 🔗

Fresh curd is what economists call a normal good, meaning demand for it rises as consumer incomes rise. As Indian households have gotten wealthier over the past decade, per-capita dairy consumption has climbed steadily, with milk consumption growing by roughly 3.1 percent per person every year, driven largely by rising rural and urban incomes. Demand has also broadened beyond plain liquid milk toward value-added products such as curd, paneer, and cheese as households have more disposable income to spend on them.

In diagram terms, this rise in income shifts the entire demand curve for curd to the right. At every price point, more curd is now demanded than before. If the supply curve stays where it was, this rightward shift pushes the equilibrium point up along the supply curve, and both the price and the quantity sold increase. This is exactly why dairy producers have flagged that demand is outpacing production growth in several years, putting upward pressure on prices.

Reading the supply curve 🔗

The supply curve plots how much of a good producers are willing to sell at each price. It typically slopes upward, because higher prices make it worthwhile for producers to expand output, cover higher marginal costs, or bring in additional sellers.

What moves the supply curve 🔗

Supply shifts when something changes the cost or feasibility of production without the good’s own price having moved. Common supply shifters include the price of raw materials and labour, the number of producers in the market, government taxes or subsidies, and technology.

Example: technology and falling prices 🔗

When a production process becomes more efficient, whether through better machinery, automation, or improved farming techniques, the cost of producing each unit falls. Producers are then willing to supply the same quantity at a lower price than before, or a larger quantity at the old price. Either way of describing it points to the same underlying shift: the supply curve moves outward, which is often described as shifting to the right or, equivalently for an upward-sloping curve, shifting downward.

If demand stays constant while this happens, the new equilibrium settles at a lower price and a higher quantity. This is precisely what has driven down prices in industries like mobile telecom and solar power in India over the last several years, even as the number of users kept climbing.

When demand and supply shift together 🔗

Real markets rarely move just one curve at a time. Income, technology, input costs, and consumer tastes can all change within the same period, and the combined effect on price depends on the relative size of each shift.

Notice the last row. When both curves move in a favourable direction, such as rising consumer demand alongside a technology-driven increase in supply, the quantity sold is guaranteed to go up, but the price could rise, fall, or stay roughly the same depending on which shift dominates. This is why economists rarely make price predictions without first checking whether both sides of the market are moving.

Why this matters for consumers and producers 🔗

For consumers, understanding demand and supply shifts explains why prices of everyday items like vegetables spike before festivals (a temporary demand surge against fairly fixed short-term supply) or why electronics get cheaper every year (steady technology-driven supply growth). For producers and businesses, price theory is the basis for decisions on production planning, pricing strategy, and inventory management. A dairy cooperative expanding processing capacity, for instance, is directly responding to the kind of demand growth described earlier, trying to shift its own supply curve rightward before prices climb too high for its customers.

This is also why government interventions like minimum support prices for farmers, subsidies on fertilisers, or price caps on essential medicines are essentially attempts to influence one side of this demand-supply interaction rather than dictate outcomes directly. Recognising which curve a policy or event affects, and in which direction, is the first step to predicting its consequences.

What do you think? The next time you notice a price change in something you buy regularly, can you identify whether it was a shift in demand, a shift in supply, or both moving at once? And between Smith, Marx, and Marshall, whose framework do you think comes closest to explaining prices in markets today?

References

1. https://onlinelibrary.wiley.com/doi/10.1111/meca.12393
2. https://www.econlib.org/econlog/adam-smith-on-the-labor-theory-of-value
3. https://www.sciencedirect.com/topics/economics-econometrics-and-finance/labor-theory-of-value
4. https://www.vedantu.com/commerce/shifts-in-demand-and-supply
5. https://ahdb.org.uk/news/indian-dairy-demand-and-supply-developments
6. https://www.imarcgroup.com/dairy-industry-in-india
7. https://www.deccanherald.com/india/karnataka/demand-dairy-products-rise-2474025
8. https://www.csun.edu/sites/default/files/micro3.pdf