Bike Valuation Request(s)

I've decided I'm going to get an older BMW (say, 86-93) as a winter
hack, probably a K100RS as I had one of those some years ago and liked
it. I want one that's in good nick with lowish mileage.

Unfortunately I have no idea how much these things are worth. I've
looked on MCN, eBay and searched the web for - there is a wide variety of prices and
the only place there is any volume is eBay. Is eBay a good place to
gauge prices? Is there any way to do a search on bikes like that on
eBay that have already sold? The only way I could find to search for
closed listings was if I named a seller.

Alternatively, is there anyone here who can give me a ballpark price
for the years I mentioned please?

any use?

Thanks but I don't think I really fancy an LT.