I think it is always proportional. When sellers negotiate with the buyers, they always set prices for every number of goods or service that you ask for. Sometimes, they may lower their prices if you choose to buy wholesale. But these are still based on proportions. For example, if you buy a single shirt, it would cost $10. Now, if you buy at least 50 shirts, then each shirt would now cost $5. There is still proportion as basis to determine the total cost.
