For 2 mounts on my 94 Cavalier (2.2L 4cyl), it cost $50. I jacked the front of the car up and placed it on jackstands. I then used the jack and an old chemistry book on the oil pan to take some weight off the engine. The old mounts unbolted and the new ones went right in. It took about 2 hours, and that includes 20 minutes to drive out for loctite when i couldn't find mine.
If you do it yourself, it shouldn't be too bad (just be careful and do one mount at a time). I have no idea how much it would cost if you get it done at the dealer.