My friends and I are looking for a shooter which we can all play online, not too new because some of us have older pcs and not more than 30$ (and less is even better)

currently we´re looking at L4D2, COD:MW1 and CSS.

which would you guys recommend, or are there any other good ones?

Halo PC, L4D 1, and CSS are the only ones I can recomend. I 've had Halo PC for years on many laptops, L4D ran fine on my past computers, and CSS is...well CSS.

Wolfenstein: Enemy Territory is great. (And free.) Although it was release in, what, 2003, it still has a very active community boosted by mods and custom maps. Which, of course, is mainly a good thing but also requires you to download tons of stuff on most servers. Luckily the auto-download feature makes things easier.

L4D1/L4D2, Serious Sam, Borderlands (Goes on sale for <$30, but may not run well on older PCs)

L4D2 or Counterstrike for definite. If you fancy a free game, it might be worth giving Alien Swarm a whirl, it's not an FPS, but it looks very good.

Team Fortress 2