It is a market value
It is worth as much as someone is willing to pay for it, that is the market value. When you price your bike, consider what person can get for that money. I just checked the Klein website, they list attitude XX at MSRP $2749, which means that store price of that bike is $2500. So person can spend $2500 and get new bike with warrany etc.
Rule of thumb is 2-3 year old bike will sell easily at 50% of their original price unless something wrong with the bike. Upgrades usually have no added value to a used bike buyer since he will be comparing the price to stockly equiped one.
Since your bike is only few month old, you are in tough spot. You spent a lot of money on a new bike, but many consider it used.
Not many people will be willing to spend $3000 for a used bike, when they can add g $300-400 to get the new bike with warranty. It is like paying 10% premium on insurance. You can also consider what other used/new bikes are selling in that range to set your price.
Hardtails in that range and quality are.
GF Zigurat($1800 new) (XTR)
Trek 8500 ($2000 new), (XTR)
Stumpjumper disc ($1400 new) (XT/LX)
Stumpjumper S-works M5 ($4000 new) (XTR)
Threrefore I would assume around $1500-1700 is a fair price.
BTW Zigurat/8500/AttitudeXX equiped idnetical. difference is in the frame geometry and paint job. I personally like the genesis geometry of GF
You can easily find out what your bike worth by posting a free classified in your local craigslist.org and see how much interest it will generate.
my $0.02
I spend to much time typing and sound like used car salesmen, I am going to ride now, dirt and mud are calling me