Rear Main Seal Leaking

So what is the Rear Main Seal? Is it on the engine or transmission? I'm buying a 89 grand marquis, and the only major problem is the rear main seal. If i have someone else doing the work for me (depending on how hard it is), what kind of price am I looking at?

The rear main seal is the engine crankshaft seal. To change it you have to remove the transmission and flexplate (flywheel).

the engine can stay put?

Usually the engine stays in. Remove the transmission to access the crankshaft. It really helps if you have access to a hoist.