How do people decide what are, to them, trivial sums of money? For example, if there's something I want on a whim and it's less than $10 I'll probably buy it without much consideration. If it's $15 I'll deliberate a bit more and probably not buy it without good reason.
I know people who make less money than I do who consider larger amounts to be trivial, and I know people who make more than I do who are more frugal. If people were rational there would be some common percentage range of disposable income that would be considered trivial, but I'm not sure if that's the case.
So how small does a purchase have to be for you to consider it trivial? I know some marriages where the spouses have an agreement to discuss expenditures of more than, say, $100, but that seems like too high of a threshold to qualify as "trivial" to most people.