For those concerned about some teams actually spending a lot of money on players, ...
Maximum dollar limit of the nine players on the field. Let's say $100,000,000 in annual pay.
Suppose the Yankees have $95M out there with a marginal pitcher who makes very little. If the Yanks want to bring in Mariano Rivera, who makes $15M, the Yanks must remove at least $10M from the field.
Nick Swisher makes only $7M, so he won't do. How about removing Yankee captain Derek Jeter? Jeter makes $22M. Heck that should provide flexibility for the rest of the game.