The rear main seal is replaced by removing the transmission and the flywheel for access to the back of the crankshaft. Asking "how much" first only directs you to someone who likely has the least experience repairing this concern. Before you go that direction you really need to know for certain that this is what is wrong, the leak could easily be the engine oil pan and/or valve cover. It could also be the oil sending unit, or even simply the oil filter or seal. All of which will result in oil leaks appearing under the car at the rear of the pan, often times because of airflow pushing the oil there.
How much oil are you going through? Consider as well that rear main bearing wear could be the actual cause of the seal failing, and that cannot be seen from the outside where the seal itself is at.