Michael Williams puts up some thoughts as to what constitutes a "trivial sum" of money:

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.

It's certainly not the case with me. Much like Michael, my threshold is about $10, including tax. It actually remains there whether I'm making in excess of $30k a year as I did with UFC or if I'm unemployed. I'm not sure why it's at $10, other than just the feeling that if I'm giving someone a $10 bill and I'm not getting anything back, it seems like at least a modest investment to me. Obviously, I do spend more than $10 on a lot of things, but it ceases to be trivial when I do.

I actually have another threshold at about $50, when it goes from non-trivial to serious. If I'm spending money towards something that's important to me (say my computers or a gift), but it goes over $50, then I'm not only going to give it a second though, but a twentieth as well. When I went to my first anime convention, there were so many things I wanted, but very little I could afford. When I went to my last convention, I could afford it all yet I talked myself out of actually buying anything because everything I wanted (box sets, mostly) just about was over $50. Even though the money was there and I knew it would be my last chance to buy these things, I really couldn't justify doing it.