If it is the rear main seal, you shouldn't be worried about the price of the seal. You should be worried about how much labor you will be paying.
Get under it, clean the oil pan and block off as much as you can and a couple of days later, get back under it and look for the source of the leak. More than likely, it is the oil pan gasket and not the rear main seal. If it is the oil pan, the hardest part will be getting the old and probably quite rusted exhaust unbolted so you can remove the oil pan. If it wasn't for the exhaust, you could do the pan in about an hour.