Yep. IIRC, to replace the exposure lock button, the mirror box has to come out. So, you're looking at ~$158-$170 in labor right there. Add a foam job, and you've an extra $10 or so in parts. It adds up. If the camera was an F3P, F3/T, F3H, or Limited, I'd say go for it. Otherwise, find a different body. In fact, I have a pretty clean F3HP that I'm selling right now. I don't think I can get as low as what BradleyK paid for his, but I am open to offers. May '95 build, btw.
If you are attached to the camera, there are a couple shops here in the PacNW that are good with respect to servicing F3's.
1) Camera Clinic, Shoreline, WA - 206-367-2440. Hours are 10-2 PDT (or 1-5 EDT) Tues-Sat. My F3P is currently there, being overhauled - major project with this one, but...
2) Northwest Camera Repair, Everett, WA - 425-252-0932 or http://www.northwestcamerarepair.com. Hours are 10-5 (1-8) Mon-Fri. Repaired a previous F3P of mine, then a Nikomat FT2, and just recently fixed up a DP-12 finder for my F2AS.