Taxes are often based on profits, not sales. Profits are reduced by costs. Costs can be marginal costs (eg. the money McDonalds spent buying the contents of you

Taxes are often based on profits, not sales. Profits are reduced by costs. Costs can be marginal costs (eg. the money McDonalds spent buying the contents of your sandwich) but they can also be fixed costs (eg. the cost of constructing the building housing the McDonalds you're buying from). Whether you buy the sandwich or not, the fixed cost remains the same. So because the taxes paid end up being based in part on the fixed costs, it isn't really possible to determine what amount of tax is based on the sandwich you just bought. It depends on how the cost of the building (for example) is apportioned amongst all the sandwiches they sold. Along with everything else they do that makes money.