i know my friend bought a Maxima from there and it was an 09, he bought it for 16,700. I forget mileage, but the carmax specialist did say that their evaluation is more precise then any other company here check out what they do to evaluate your car:
How Our Car Appraisers Determine The Offer for Your Car
I think with a phone call you may able to get a ballpark figure by describing your car, but to get an exact amount I would say bring it in. Also do a quick KBB check up while you are at it.