The reason Kravenwolf is saying that it would cost more (Yes, that's what he meant) is because he asked me about Co-op a while back. I laid out for him what would have to be done to FPSC in order to get it working to a decent effect... basically, if you paid $20 per change (Not at all inexcusable) you'd be paying a lot.
However, if you're looking to make a cheaper co-op (not as glamorous, but still co-op in FPSC) there are a few things that would need to be changed. It would still be client-host, though, and you would be relying on the way FPSC does things already.
Basically... force a second entity into the level with "half" of the MP system active.
The main things that are holding this back are:
- No Client-Server setup
- Dynamic entities are not kept up-to-date between clients in MP
- TGC couldn't care less about MP in FPSC
The one and only,