If you're thinking of hiring, getting a general handyman will likely be quite a bit cheaper than Best Buy's people. You don't need any specialized knowledge to fish wires or patch drywall, and it's not electrical work. Furthermore, in-wall speaker wire is cheap as mud compared with what an AV shop will try and sell you.

You might consider a professional if you're doing multi-zone systems or some kind of crazy remote control setup, otherwise the work you need done should not run you $500