I need help with plastics

[mechanicalbrain]

I've been planning my summer project and I started getting into material selection. The main thing I'm having trouble with is picking plastics, something I really have no experience with. The dimensions I'm looking for are four 1'/1'/1.5"-2.5" blocks but I'm not adverse to cutting some plastic sheet as I'm going to have to machine it anyway. Some of the factors I'm keeping in mind are
Cost: I'm on a really limited budget. I'm looking at paying no more then $60-$70
Strength: While the material should be reasonably hard I'm more worried about it not being brittle as it will have to deal with a moderate amount of torque.
Machinable: I'm not too worried about this but it should be reasonably easy to work with.
Weight: It has to be decently light weight.

I think that's about it. If it helps I need the plastic for wheels. It won't be used on anything but carpet or tiled floor and at relatively low speeds. Thanks all for any help.