The price I pay of my neighbour not killing me if he feels like it is not killing them should I feel like it - that's the nature of society. We also both fund a police force that will attempt to prevent and punish crime. It doesn't seem hard to determine at all.
Markets work differently from societies - the price consumers pay in a market is the price the provider asks for, for the product the consumer has chosen. The provider could set an unreasonable price if he wishes, but the consumer could always decided that the value provided isn't sufficient and not buy the product.