I think you've probably shot yourself in the foot by bringing money into the discussion too soon! This would have been a good place to build up some goodwill and get your name out there associated with these people.
You might want to try this link:
The Stock Photo Price Calculator
It's mostly aimed at pricing for non-exclusive corporate clients, but it could be a place to start from. (IMHO even their lowest prices seem to be a bit excessive for what is essentially a non-profit organisation.)