I had a similar situation recently in my (non photo-related) business. The customer didn't want to pay the price I felt our service were worth. At the same time the money was not that important to me - it was more this "hey, it's worth that much"-thing. So I suggested to give the difference to charity. They got my point and were happy to do this - good publicity for them, too. So maybe this might be something for you and them as well. A nice story of the local newspaper supporting the local hospital (just an example) looks always good in print. Just my 0.02.