There is no simple answer there. A 'fair price' is whatever you're happy to pay for them, and whatever he is happy to part with them for.
Painting throws any sort of accurate guide out of the water, since the fact that they are painted can be a bonus (saves you doing it, or if the painting is good) or can be a nuisance (if you're wanting to repaint them yourself anyway)... so being painted can push the price up or down depending on just what you want from them.
But I wouldn't consider 50-75% of retail (or at least discounted retail

) to be excessive. Less than that is a bonus.