k, i came kinda late on this topic so bear with me without quotings.
it sounds like an 'ok' idea, but i have a few things to say.
1. players alone make a lot of lag for people, wont monsters + players make it a HELL of a lot worse? and thats the least of the problems that could occur
2. what would the map look like? can u make like a floor of it? cause...a map like deathmatch would be too small for a place like this...infact all the current map sizes would be too small, and another alpha styled will just be way too laggy
3. like jesse said, who would get the money if we both killed it?
4. at the start you said you cant player kill, then you said you get money for it :S
opinions
1. sparring and killing monsters dont go together, personally i think an arena for sparring should be placed in the deathmatch dropship, but thats just me
2. i do liek the idea, dont get me wrong, but as for the 'monsters' u stated, i dont think they would really fit in :S
mutated sharks? (for 1) dont fit zone at all, aliens however i could see
3. i think this should be the solo kind of mode, itd be much better, or maybe maximum of 2-4 people and you split the reward at the end of the 'mission'
4. maybe you have to get through a certain amount of monsters (maybe like...125 per person (so if 4 played, you would all have to kill a total of 500 monsters)) and as for reward, how about 100-150 bucks a person? thats not that much, still makes gettin money time consumed, yet you dont return empty handed from doing this. and the key to this prize is, if you get damaged a lot, basically that money goes toward med kits/cookies (WHICH WILL STAY

).
like i said in another thread, post all ideas anyway even if you dont think, good ideas may spring about, in all...i like the effort here
added: btw, aliens instead of the 'monsters' u claimed would be a big opinion changer in my view