You will receive many different opinions on this, much like asking "What is the best Gyro" (see this: [url=http://www.runryder.com/helicopter/t519271p1/])
When I'm looking to buy, here is how I go about figuring a fair price to pay.
Take the heli, and add up all the prices for the parts new. Using basic pricing, no combo deals, or manufacturers discounts. Take this number and multiply it by .6. That's right $.60 on the dollar. Don't add anything for spare parts, unless you have a full ton of them. If you have an extra canopy, that is not factory (They need to be Canomod, fusumo in perfect shape) and NOT painted by you, add 60% of the price of that.
This seams to usually be fair and I can buy most helis at this price.
Oh, this only works on current helis. Try to sell a Trex 450 V2 like this, and you will be holding it for a long time. But a Protos, or 450 Pro (both current models) you will move it in a week or two, and get close to asking price.
And I didn't write this to get a steal on your heli, I already have two Proti (?), the second got used at 61% of new list price.
Don't forget to add pics, and lots of them.
Screw It, I'm gonna fly
Synergy E5, E5s and Logo 800 xxtreme
vControl and Neo