Just brainstorming a bit here for our general use of credits. In this other forum I hang around there's this "Risk esque" forum game where the players try to take over each others countries.
Players decides their actions (which can be as arbitrary as they want) and sends them to the GM. All actions have some kind of effect, but they also have a chance of failure. How big the chance is, I guess depends on the level of effect the action will have (decided by GM I guess). But the more credits you "invest" in the action, the higher chance it will have to succeed.
This brings players to the decision of parting with more or less of their cash in order to succed in their actions (whatever they might be).