I have found this question being asked often and its always about work done that is not a primary source of income. If you are selling photographs full time or part time you should quote
An hourly/daily rate
Re-touching by image cost.
Additional expense- printing, books, CD's ect.
Don't forget when you work out your hourly rate to consider - Equipment Depreciation, Insurance in particular. But all or any costs associated with your work.
If you are happy just to do it for fun, thats fine, but if you are going to charge then cost it properly otherwise it could be tears at bedtime if someone drops beer over your camera, "oops sorry" and thats $3,000 lost on a $5 job.