I took an R6 frame to Mackie, located in Sierra Madre. He did an amazing job, is incredibly easy to work with, and if I ever have the misfortune to need frame work done again I'll be going to him. He charges $100 to check a frame and prices go up from there if additional work is required depending on what you need, the $100 is applied to the total cost of repair if applicable.
The other option is Dr. Jon in Anaheim. He has a website you can check out here:
http://drjohnsmfs.com/
Dr. Jon's pricing is listed and I hear he does good work as well
The most important piece of information I can give you here is this: Dr. Jon was trained by Mackie.