Not enough information. If the wiring is old enough that there is no ground at all in the branch circuit wiring, then the presence of the GFCI (with the appropriate labelling) is not an inappropriate solution.

If there really is a branch circuit ground but for some reason it is not connected properly, you've got another story.

Home inspectors are generally blithering idiots when it comes to code issues. All they know how to do is stick their little three light tester into the sockets. I presume the $500 came from a real electrician? Can't vouch for the reasonableness of the price without knowing just what needs to be done.

Your best bet on checking price reasonableness is to:

1. Get multiple estimates
2. Get references, preferably from people you trust, about the prior performance of the contractor.