If y'all have good teamwork skills I'd say look into Natural Selection 2. It's a super addictive FPS/RTS hybrid that is incredibly fun to play. It's kind of like Starcraft meets Alien, and sets a very fun atmosphere to play in. Gameplay is strictly Alien vs. Marine PVP, but is very balanced. The community is overall really friendly, and love to help new people. On top of all this I'm pretty sure you can get a four-pack for you and your friends for like 20 something dollars.