To put the comparison simply, NVidia designs graphics cards for non-gaming as well. Their cards are used pretty much exclusively for almost every professional use. Because of this, the technology they use has a much higher standard for accuracy and reliability. Their products are of higher quality than AMD's. However this causes their price tags to carry the weight of all of that peripheral use. The prices are higher on their cards because that's how they cover their considerably higher R&D costs.
AMD's workstation cards haven't been competing with NVidia's very successfully however, and they know it. As a result, they've been focusing on the gaming market and have done so phenomenally well. Their low-end and mid-end cards pull great results for their budget. and in a budget build you're going to end up wasting money if you go NVidia. On top of that, they allow developers to utilize their Mantle technology which lets the programs be designed to work right with the hardware. Mantle technology is likely going to kill the one technical advantage consoles have over PCs now, which should speak to Mantle's effectiveness. Essentially, Mantle will give you an extra couple frames in those high-end games, letting your lower end AMD card compete with higher end NVidia ones. This advantage however is only present in certain games, so make sure you check whether or not your favorite companies are planning on utilizing Mantle in their future games.
Any graphics card comparison below $200, AMD's contestant will most likely win. $300 and up, NVidia is almost always the winner. Between $200 and $300 is your grey zone, where their price/performance starts to overlap.