Short and straight answer:
It all depends on the asking price and how much you're willing to spend for a KRS/GT.
Personally, for a '98 with 50K miles and very well mantained I would not spend over $5K, unless of course it is loaded with accessories, which in that case I would go up to max $6K.
These days you can find a good 2001-2002 with less than 30K miles and in excellent conditions for as low as $7K from a private person.
BMWs bikes are good machines overall but they have problems too, and when problems occurs they are very expensive to fix.
'00 Valkyrie I/S
'03 KRS - Sold in 2006
'04 R1150RT - Sold in 2006
'03 K1200GT - Sold in 2009