If the bike has not been serviced in the past 3-4 years and you don't know how to perform the service yourself, you would be looking at about $2K if you have a dealer to do everything you have been suggested on previous replies.
Keep this factor in mind when you negotiate the price.
Unless the bike comes with extra accessories and it is in excellent conditions I would not pay more than $8500 for it. That would bring your " ready to ride" total cost to $10,500 which is already kind of high for a K12GT even with only 3500 miles.
'00 Valkyrie I/S
'03 KRS - Sold in 2006
'04 R1150RT - Sold in 2006
'03 K1200GT - Sold in 2009