My main concern with this is that, depending on the objective, it could cause people to play or act worse as they're "trying to get their RP." If you encourage any sort of behavior in-game outside of "kill the nexus," then that inherently damages the integrity of the Ranked ladder. I think the best example I know of is in a team-based shooter (Team Fortress 2 comes to mind) where you've got the tryhards wanting to win, the guys who just want the achievements and spend their whole time being counter-productive, and the guy who, for some reason, is going for the high KDA, even if that doesn't matter for the sake of the match. A monetary reward may be slightly different than Steam achievements, but I feel that, especially in Ranked, it could possibly create more issues than it solves.