It really depends on your local market. The shops around here quoted me anywhere between 250-300 for my 70 Rupp Roadster -- and that was using my powder on my pre-sandblasted and cleared frame.
A shop in Knoxville, nearly 2 hours away, quoted me $100 using my powder on my pre-sandblasted frame. From what I'm seeing, that's a really good price.
Do your homework and call around for quotes. You might also want to check out their quality of work before you drop it off. The first time I had something powdered, they didn't clean it properly and the old paint underneath caused the powder to bubble. It looked like garbage.