commission cost

One thing to judge is how long does it take to do your paint work. Per hour basis on time spent painting.

Custom painted heads can often be sold for high sums of money, more than a couple hundred $USD. A look through Ebay shows quality on custom painted sculpts is all over the place, so you can judge your work against some options like that and price accordingly.

At the end of the day, you can price it as you deem fair, as it's your work.

What cdn_rhino said.

Also, be aware if you start charging high sums of money, you may not get the business you think you will. Very few people in this hobby can afford to spend hundreds for just a paint job, but they might be willing to spend a hundred or less. Darren Carnell (in the Uk) has skills that far exceed Hot Toys, but he charges $500 USD per head sculpt, and it takes years to get your stuff back! I’ve seen guys do pretty reasonable work and only charge $60-75 USD for their work, and Their turnaround is less than a week. If your interested in honing your skills and want more work to get that practice in, lower pricing to start may be a better option — as your skills rise, so can the rate you charge. The choice, of course, is yours as stated.

Rhino and Striker, thank you very much for your advices