[Supaman771]

Someone host a 2 hour ULMS and loan me an Mp5 so I can make 10k?

Seriously though...

Add the thing like with hp levels, where you have to kill one person then another before re-killing the first player (prevent abuse).
Add an extension onto kill messages 'You killed ______ and received $_!' (to keep track or whatever idk)
Add some kill streak bonuses (if a player gets like 3-5 kills in a row and then you kill them... 'You ended _____'s streak and recieved $_!') would give a higher amount (random 20-50?). Also if you do this add a 'Kill Streak' counter to the GUI :3.
Add a 'Random Player Bonus' like TSA said, where the NPC would mark 1 random player with a Bonus so when they die you get a large sum (250-500) or whatever... mark a player like every hour (or half) or something.

I haven't actually played so I don't know if you added any of these things..
High5!