From a perspective of myself with a hobby shop, if I was buying them to resell them as a used kit, I would probably offer $5 each, with the intentions of selling them for $10 each.
That being said, there may be a few rare ones in there that might be worth slightly more, but even the so called "limited edition" kits that Revell put out, they still produced 15000 plus kits, which doesn't make the kit all that rare. 1000-2000 kits is what I would consider rare, 15000+ kits is a normal run (at least back then).
From a personal perspective....honestly I probably wouldn't go much over $7.50 to $10.00 per kit. Most can be found on Ebay at around $10, with the exception of a few. Offer him $500 for the lot and see what he says.
I would want to have a look at what he has, check all the boxes to make sure all the parts are there, and the decals are still good. If the decals are trashed, then no more than $5 for a kit with trashed decals.
Also take into account how many (if any) have been started. Bottom line, if all the kits are perfect, would say now more than $700 for the lot.