I have been doing some retouching for free over on Model Mayhem.
There is a thread there where people can post a picture they have been having issues with or that they cannot themselves fix.
People who want to give it a shot can do so, then post the results. The benefit to the photographer is obvious, they can have several retouchers working on their shots for free and use that shot in their ports as long as they give credit to the retoucher. The benefit to the retouchers is that they can then use the image in their own portfolio, again as long as the credit is given to the photographer. Works fine.
The down-side is that if your shot is not particularly interesting or more difficult than normal, often it will be ignored in favor of shots the retouchers think would look better in their books. No one HAS to touch the shot and often, these shots sit ignored.
A side benefit for people like me, is that I get to play with a bunch of shots, with a whole host of problems and compare my attempts against those of people who do this as a job. Trying to recreate the end product of a talented retoucher makes me learn new techniques.
Now to the crux of the matter. Even though I have never billed myself as a professional and do not have a retouching portfolio, I enjoy a bit of a rep and am getting messages asking me to do retouch work on photos, asking that I work with specific photographers on a regular basis etc.
Since there is no benefit to me (IE advertising in a port etc), I am going to start charging. It will either bring in some money or it will end the messages asking me to help, either is OK with me.

How much? Is there anyone here who does this professionally? What is the service worth? Flat fee or time spent?

