As a general rule, if I'm even interested and it's under $5 I buy it. Anything over $5 has to be something I want--for instance, Rayman Origins is $15 right now. I'm interested, but not enough to bite at that price.
For games I own, I do the obligatory DLC check and pick up the extra content.
Then I wonder if games can go lower. Hard Reset, which is high on my list, is 75% off right now. It's $3.75. Is it really going to get an even greater discount? I'll wait, but damn, even at that price nobody should pass on it.