I have found carmax to be about 10-15% higher than prices obtained from traditional dealers for identical vehicles. The asking prices are similar between traditional/carmax but the purchase prices are much different.
Carmax really just charges full retail minus a few hundred bucks. That is not a good deal at all. On top of that they pay the rock bottom price for trades.
Some people are more than happy to pay the extra 1-3K for the warm and fuzzy feelings judging by the amount of carmax tags I see on cars around here.
Carmax is the worst deal on cars than any dealership in my town.