KBB.com's values ..
Trade in Value: $4200
Retail Value: $6100
NADA Values ...
Low retail: $4,120
High retail: $5,420
If they've added aftermarket parts - add up the total cost of the parts new, then divide in half. Add that number to the above figures. That's about as close as you'll get with values.
Just an example, there's a 2006 Thruxton listed on Craigslist in Austin, TX right now. 7K miles, bar end mirrors, extra seat ... with an asking price of $4800.00