I usually prefer buying games cheap on huge sales on Steam, every summer or every winter. They have a grand summer or winter sale every year, usually with an interesting event to accompany the sales. So, right now I am waiting for that. My price range is lower than 40$, I don't like spending over 20€ on a single game, unless I REALLY want the game and if it's a game that has recently came out. Most I spent on a game was 60€. I pre-ordered the first Black Ops, and I regretted that, and I bought the Bad Company 2, which was an awesome game, and I do not regret that purchase. Oh and, I pre-ordered Skyrim as well, don't regret that either. Other than that, I didn't really buy any other expensive games on release, only on huge sales.
My advice to all is, wait for the Thanksgiving sale, it should come soon, and if there is nothing on that sale you like, wait for the winter sale. It will be W-O-R-T-H it, trust me on this one.