I think the prices you mention for the R100GS and R80G/S are too high, those prices would be for very nice examples, exceptional condition. Sounds more like a hopeful asking price, not many of either will get that kind of price.
Are the above airheads going more than an R1100GS or R1150GS, I suspect yes. I wouldn't want one of those over weight pigs plus all the electronic stuff and FD issues, yeah the GS has the same FD issues.
airheads are by design easier to maintain/ cheaper, but require more tinkering, but if an oilhead requires work it's gonna bruise the ole pocket book big time, plus more of a PITA to work on.
I'd guess prices for a decent/ average R100GS to range from $3,800-$5,700 maybe a little more. Of course a low mile cherry for around $7,500, a G/S about the same.