Susan,

When I figure fees I do what you seem to be already doing. Assess likely size, reasonable fee per person, etc. It's a good idea to set a fee that would guarantee that you do allright for your time, but where the place holding the workshop doesn't lose it's shirt if attendance is not what was expected. Also, a factor to consider, how much value are they placing on you coming? If they aren't willing to cough up at least $100.00, they probably aren't going to value what you do (not always true, but this society measures a lot of things by money and not other considerations). Psychologically speaking, if you aren't willing to think your work is of value, why should they?