Multithousands if the real thing. I don't think I've seen a "for real" WWI "survivor" in person in ages and ages. And not too many reconstituted ones either that weren't glaringly obvious. Even those aren't cheap.
I'd suggest that you and your friend settle on a price that y'all both aren't too happy about. And keep the teeming hoards well outside the deal. That is if you both want to remain on best terms!