Shared Budget Constraint

principle

Complements cooperate in use but compete inside one total budget. Making a required complement cheaper can shift willingness to pay toward the focal product.

A shop can make customers willing to pay more for shoes by cutting the price of socks. A ₹500 pair of shoes and ₹200 socks consume the same ₹700 footwear budget; cheaper socks leave more of that budget available for the shoes.

Partners at checkout, rivals for the wallet

Complements raise one another’s usefulness, but usefulness is not the same as budget independence. The buyer evaluates the cost of the usable bundle: shoes plus socks, not shoes in isolation. Reduce the unavoidable complement’s claim on the shared budget and the focal product can capture some of the released willingness to pay. vendor-lock-in reverses this opportunity when a proprietary complement prevents customers from seeking a cheaper alternative.

Where it shows up

The ₹700 footwear decision

The shoe and socks are consumed together, yet each takes a slice of the same footwear allowance. Lowering the socks from ₹200 changes what ₹500 shoes feel affordable—not because the shoes improved, but because the complete purchase became cheaper.

Freed budget is not guaranteed revenue

This model requires a genuinely shared constraint and a complement the customer regards as necessary. A lower complement price only creates room; it does not prove the buyer will surrender that room to the focal seller. And if compatibility makes the focal layer interchangeable, pricing power may migrate elsewhere instead.

Price the whole usable bundle

Tomorrow, list every required purchase surrounding your product and estimate the customer’s total category budget. Instead of asking only whether your own price can rise, identify the complement whose cost could fall—and test whether that saving increases conversion or willingness to pay for your product.

Episodes that teach this

Substitute Goods & Complementary Goods | Interesting Economic Facts 1,058 views
A 500 rupee shoe plus 200 rupee socks means a 700 rupee footwear budget; if socks get cheaper, people are willing to spend more on shoes.