I think I paid about $175 for a new bolt and about $200 for a semi-auto lower. My plan is to eventually have the lower modified so it can run on my 76.
Autoweapons.com had a ton of S&W parts a few months back. Their prices were fairly steep, but I managed to score a few things I thought were well worth the price.
I've thought about looking for somebody willing and able to reverse engineer the bolt.