> Very different. In the case of a single supplier - wouldn't it make sense to pay everything you own ( minus what else you need to live ) to survive?

That's assuming you get to negotiate with every customer. If it's one price for everyone then while what you say is true for each person's max (ignoring the complexity of people looking after others, the same logic applies of "everything you can afford") that's different for everyone so the optimal price can vary.