Using your money analogy, what I'm saying above is that if you expect the item to cost $20, you will go to the shop with only $20 or $25 in your wallet. So you won't buy the item if it costs $100.

This theory is compatible with willpower being expendable - obviously, you can't carry more cash in your wallet than the total amount you own.

So it is a more detailed model, in which you can be short of money in two ways: you can't afford this at all, or you didn't think to have that much cash on you when you left the house.