I paid $150 for my G5 w/ attachments, which I think is fair based on local ads for G series vacs. Paid $20 for my G3 which has broken fan, dirty looking, but has attachments. $25 for my Heritage 84 with no attachments, but it's in pretty nice shape for it's age. I think $75 is fair for a Ultimate G only reason I wouldn't buy it is because I have the G5, and I don't want to collect the Generation series.
I go to thrift stores once in a while, as well as yard sales, and I do not come across many Kirbys. If they were as common in my area as they seem to be in yours, I'd likely have 10! lol
The Ultimate G is not old, but I only say that because it's almost the same thing as the Sentria (which people might think is old too!). My wife is one of those people, just because it's not plastic with a clear cylinder to see the dirt. Kirbys last a lifetime, a 10 year old Dirt Devil I would say is "old", not vintage old just "time for the dumpster" old...