For a mere 560MSP you can try out co-op arcade mode which seems like a horde mode.
“Deluge co-op arcade mode where you defend the city zones against ever growing waves of Pacific City bad guys. Try it alone or with up to four players on Xbox LIVE for ever more demanding battles.”
It also includes16-player capture the orb with 6 arenas to wreak havoc in, 10 new achievements, and avatar awards.