I know there a lot of servers that have survival Games when players team up to kill mobs, but let make it interesting.

Have Chambers Set up, possible 20 x 20 or 40 x 40 about 5 - 10 chambers. now have Waves of spawns enter chambers to attack players, and have players fight for their lives. last player Standing will get Award money just like in SG.
Thinking a head
Player place Valuable items in chests befor entering, then supply them with food, weapons and armor in chest inside chambers to fight off mobs.