It'll depend on several factors - accessibility of pathway being a big one. I added a dedicated 15A circuit at our old house and since it was a single-story slab-on-grade I ended up running exposed conduit on the exterior of the house. At the same time, we added a receptacle outside on a separate circuit in the same pathway. It's not what one would call a "pretty" installation, but I think it was around $250 in 2002. That's the bottom end of the range, since it was a relatively new house service with spaces in the panel (replaced the original 60A service in 1999 shortly after I bought the place with a 200A service), a relatively short run, and entirely exposed surface-mounted conduit. If you will be doing anything through a crawlspace or attic, I'd expect the price to go up. The $500 is probably closer to a reasonable ballpark without knowing more about the specific situation.