How do you appraise amateur art?

I have to get an inventory of my parents' estate, including some paintings that my mother did in the 1970's. She was an amateur artist, and not incredibly good, but good for an untrained artist. The lawyers say we have to have a monetary value for every piece. To us, they are priceless, and will never be sold, but to a dealer they would probably be worthless. How in the world, short of paying an appraiser to look at them, would we put a price tag on them?