Well I'd start him at 50 quid!
Then wave a few more notes in front of him if he doesn't bite.
He'll be happy with cash in his pocket for xmas stella, and you get the nice looking bike to play with!
If you just want to buy it then make a few quid in the resale, then that's up to you and your conscience!
Some things have a clear value, others, well if you can get a bargain that's what makes this hobby fun.