I usually charge by the hour.
My rate is 75$ per hour. I can go down to 65$ per hour when there is a lot of pictures to retouch.
And if the client ask something more complex, like something more artistic (any kind of illustration style), I'll go with 100$ per hour.
And, by example, if the client doesn't want to pay more than 100$ per picture, you just have to try to not work more than 1.5 hour on each pictures... 50$ per photo = less than 1 hour each... you can just adjust yourself to the budget.
