I'd start lower and see what he says. He ays he has no idea... otherwise he'd state a price, but I tend to doubt the "I have no idea of it's value" unless the person lives in a stone-age cottage with no internet connectivity.
Down here in the lower-48, where film still lives, I got a SuperGraphic that needed a GG/fresnel for free about 20 years ago, and a ready-to-use, fully-functioning Anniversary Graphic (with flash, 8 film holders, and a case) for $25+shipping about 3 years ago
What's the worst thing the person will say if you low-ball the offer. Generally it is, "well, I was thinking of something higher"; to which you can reply, "OK, what were you thinking?" I assure you... the seller has a number in his head and is hoping that you'll offer more than that number.
A lot also depends on the condition... I noticed you make no mention of that.