The TARP program was originally estimated, in the fall of 2008, to cost taxpayers $356 billion. As of April 2010 the estimate was reduced to $89 billion. Paybacks from troubled firms have continued to exceed expectations. An estimate made today would likely be even lower. TARP was not funded, it was just budgeted. There is no leftover pool of money.
The BP spill is a different story. It looks like another doom that didn’t happen. Was it a disaster, yes, but a $20 billion disaster, probably not. In this case there will be a pool of leftover money because the $20 billion is funded with cash from BP now sitting in a fund under government control. It cannot be used for campaign expenses but it could buy happiness for some appreciative voters to whom it is distributed.
Why not use the excess money to better fortify the levies that failed when Katrina came ashore? It is not politically feasible I suppose. Shame.