I find bicycle blue book to be completely baseless since it can't take into account location and alternate options.
Sure, it's a good place to start, but you must take the pricing with more than a grain of salt...think a salt lick.
That pricing, to me, is nuts unless it comes fully reconditioned with new drive train components and bags and racks.
A new 520 is $1200 and spec'd really nice.
A Novara Randonee is $100p or so with coupon and is spec'd really nice.
Given my 6 bikes are between 23 and 35 years old, I am all for keeping an older bike going. But $100p is nuts.
If it fits you perfect, I would think somewhere between $250 and $500 is reasonable, depending on condition of components and frame.
Chainrings which need to be replaced are $60 total for non ramped. New tires will be $100.
Etc etc etc.
Costs add up quickly for updating.