Online Co-op Game (besides L4D)?

I'm looking for a good (somewhat cheap... maybe 20 bucks at the most) online co-op game that can be played by three people. I'm not too picky about the genre, but FPS preferably? The reason I don't want L4D is because it is too expensive at the moment, but we'll definitely be getting it when the price drops. Me and my other friend finally got our other friend to get TF2 after bugging him for 5 months, but he doesn't really it like it that much because he'd prefer something where we could all work together in like a little squad I guess you could say. I know you're probably screaming L4D, but he is a cheap b*tch and only got TF2 because we offered to pay for half while he paid for the other half. Anyways I'd really appreciate suggestions. Thanks.

Red Alert 3?

Either wait to see if Steam has a deal on Left 4 Dead when the content hits (it could happen as a free-weekend 50% off deal!) or the only other compelling co-op game I've played on the PC is S.W.A.T 4

keep your eyes on steam every weekend. Steam has already offered L4D at half price once and they might do it again with the new DLC, like they do with TF2.