A/D/S PQ20 find

A friend of mine has a couple of A/D/S PQ20 (4x100) amps that he would like to sell to me. I have no idea how much they are worth and neither does he. I have seen a pretty big swing in prices in forums over time (just found a few sales on different forums). Some have sold for a bargain price, but some for more than I would pay for a current 'decent' amp. I trust that he has taken good care of them and I am also pretty sure that he did not buy them new.

What do you think would be a fair price? These are the made in Japan ones.