Romney's campaign (not counting SuperPAC ads) only spent about a million and a half on Super Tuesday ads which I felt was a mistake as another million in Tenn and Ohio would have provided him much much better Super Tuesday election night coverage and that would have been worth 20 times as much or more.
The real question will be how much will Romney spend on winning in Pennsylvania. Right now it feels as though he is playing a game with Rick in campaigning around the state and leaking that he is going on the air, but not saying how much. The name of the game is telling Rick pretty clearly after you have your Easter weekend you either get out of the race or I will fight you tooth and nail over your home state and I will beat you. But, get out of the race and you won't have to worry about it and you will have a brighter future in the GOP down the road if you so choose.
As for what he should be willing to spend on winning Pennsylvania if Rick stays in? Anything he has on hand and if he wants to go to the piggy bank for more I would do that as well. He beats Rick in his home state the race ends and Romney very much needs the race to end in April to get ready to face an already ready Obama machine.