A fair price is ___?

I'd like to buy a mate's F+F: a blue white and lime colnago titanio 1996 used but in good condition. He wants 500 and expresses that hes doing me a favour. I feel this is ruining the friendship, or am I being unreasonable?

Yes, you are...

http://www.bikecult.com/works/archive/colnagTZ.html