I had an old frame powder coated about a year ago. It was about $125 or so for one plain coat, including stripping off the old finish. I paid $50 extra for a metallic/candy finish, which looks great.
If your bike frame has steel lugs, you might want to make sure you're going to a powder coater who is experienced in doing bicycles. If they put the finish on too thick, you can lose the detail around the lugs. An experienced powder coater will know this.
If the bike is really special, you might want to consider shipping to these guys; they have a national reputation...
http://www.spectrumpowderworks.com/spwcontact.html