I buy games that I want to play, that are within my comfort range for game prices. I don't worry too much about what those prices are. Over time, as my collection has grown, the price I'll pay for a game has gone up- the logic being 'I can buy 3 games of middling interest for $10 each, or I can buy one game I really want for $30.' (Granted, the number is notably higher than $30 these days, but you get the point.)
I also buy things that I think will add a unique, if not necessarily 'fun' experience to my collection. Especially when it's cheap- I don't spend a lot of time with Babysitting Mama on the Wii, but a dollar for a game you control with a giant plush baby? I definitely don't have anything else like that!