Overall after everything is weighed up NVidia's brand is better in the market. They thus get to charge a premium for their product based on prior results. Looking at any one particular market position at a moment in time and doesn't necessarily reflect that longer term picture. Most buyers especially in the $100 market aren't as knowledgeable about which card is the fastest at that moment at their price point. Even that measure is pretty variable depending on the games you choose.
For years I have been buying AMD cards because they had better price/performance, now I am on NVidia after having a raft of issues and put simply NVidia's drivers are much better. I think honestly 680 v 7970 its worth the premium, especially for SLI v xfire users. Down at the mainstream end of the market I have no idea who beats who, but I bet the problems with power saving impact AMD's low end cards as much if not more than their high end.