Valuing my friend's iMac

Trying to buy my friend's iMac for my little bro for Xmas.

Model Name: iMac
Model Identifier: iMac9,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.66 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 6 MB
Memory: 2 GB
Bus Speed: 1.07 GHz
Resolution: 1680 x 1050

He said it's a year old with some Applecare left on it.

Where can I go online to see how much this is actually worth?

I tried finding a similar iMac to his on ebay but the prices varied so wildly I couldn't be confident.


Thanks

mac2sell gave 400

ebay is saying between 600 and 800 for this one specifically, varied by condition