If you're looking for something more along the lines of a couple bucks... before I had a lapinator... I just used a school binder. It managed to let the air flow and provide a nice angle of inclination, at a next to no cost (if you already have a binder that is).
I use a piece of wood from an old shelf. Works great, and it was free.
It's kind of long, so sometimes when I want to game with leisure, I sit in my bed with my back leaning on the wall and use it as a sort of lapdesk. Fits my laptop and my mouse perfectly, and I like the satisfaction of fragging someone from bed.