Friend has artwork that she wants to sell. need details?

she does some really good anime drawings and paintings and we have a store that specializes in that here. she knows the owner and figures he could sell some of her artwork in his shop. i was wondering if anyone had tips on how much to charge, such as an equation using the price of materials plus time invested, etc. i have seen some of her artwork and it is pretty amazing. please no rude or stupid comments. but if you are looking to buy....