The Oil Pollution Act “limits the liability of responsible parties for offshore facilities, such as the Deepwater Horizon facility, to all removal costs (i.e., direct cleanup cost) plus $75 million and other language places a limit of $3,200 per gross ton”.
This gross ton number tells you why BP has been so cagey about accurately quantifying just how much oil is spilling, has spilled, or will spill. Expect lawyers to argue about this for the next few decades. With an estimated spill of 4.9 million barrels BP has approximately $2.5 billion in total liability. (See here for more info and good conversion factors).