I miss out on a lot of good games as well because I don't like to spend more than $10 on downloadable games like these. I made exceptions for the two Serious Sam games on XBLA, but that was because I played them in the past and already knew I liked them. But other than that I either wait for a sale or just skip the game completely if it happens to go on sale at a time I can't afford more points.
That said, I own Gatling Gears because I won it in a contest from GamePro. It's a fun game, but not one I would have paid $15 for. I don't blame you for waiting for it to go on sale.