So I've done a handful of commissions now. After some time, research, external advice, and practical negotiating I have come up with what should be pretty fair pricing guidelines.
$10 for a single picture with a simple scene and a single character, plus $5 for each additional picture with the same scene and character.
$15 for a single picture with a complex scene and/or up to three characters, plus $6 to $8 for additional pictures of the same scene and characters, with price based on complexity of the changes to be made between pictures and the render time of the images.
Images with very complex scenes involving more than 3 characters are negotiable.
Surprisingly enough, out of the commissions I've done in the last couple of months, only one was for a single pic. The rest were either for progressive scenes/story images, or complex scenes with multiple pics from multiple angles. Since the most work intensive part of the process is setting up the initial scene and character(s) I decided to charge significantly less for additional pics, since they involve a lot less work on my part even though they take just as long to render and postwork. I can (and have) done commissions with adult themes and nudity, but require a statement of age, and that you're legally allowed to purchase such materials where you live.
My process is pretty simple. I accept payment in USD thru PayPal. If you want to hire me zip me a note with a description of what you'd like, and I'll either decline or accept the request and provide a quote. If you agree to the pricing I'll start work on the image(s) and occasionally post thumbnails to my stash and link you to them, so you get an idea of how the final pieces will look and can request changes before I put too much into the scene. Once the full commission is done I'll post final thumbnails to the stash and link you to them, and then you send payment. Then I'll email you a zip file with the full image(s) to the same email you paid with, and also post the full image(s) to my stash where you can download them from anywhere you can access your DA account.