Lesley -- what Bill says. I've made a living on RM images since 1992 and the Getty pricing is the easy way without explaining it all. You might want to check your library for a book - Sell and Resell your Photos -- by Rohn Engh. Also, search online for Dan Heller who has an extensive website on the photography business, though it may be dated material his opinions seem timeless. Hope this is helpful to you.
EDIT: Dan Heller's website is, of course, -- http://www.danheller.com/