The simple answer is, "What the market will bear." Browsing eBay over many years, I have seen folks pay obscene cash for mediocre paintjobs. Some people have more money than they have time or patience, and they are prepared to PAY for it.
My rule of thumb for a STANDARD/SPEEDPAINT paintjob is to charge what the owner paid for the miniature itself-- but that's for GW pricing, and it is definitely on the low side. Need 10 Space Marines painted? You paid $50 for a squad of 10? Fine, $50 for the paintjob.
However, I am a very fast, competent painter, and I think it's a fair price for the amount of work
I personally have to invest. I've never been much of a gouger.
For a single Reaper-style fantasy miniature, painted to a high (if not expert) level of detail, $15-20 seems fair. I suppose an expert paintjob could go 30-40, and if you included sculpting and conversions to personalize the figure to the buyer's character sheet, 50 dollars up to... well, as I said before, "What the market will bear."
Wulf
P.S. If you are as curious to see some of my Speed Painting work as I am to show off, check out these links:
http://www.batreps.com/hive_fleet_project.htm
http://www.batreps.com/auc-bevier.htm
Personal "Speed Paint" Work-- a couple of armies I painted in way too little time to prep for tournaments:
http://www.batreps.com/gbd-sm.htm
http://www.batreps.com/gt-orks.htm