Not only do you get compatibility, but you also get speed up some time 4x faster in certain programs due to Driver Level features enabled only for quadro cards.
If you are not doing 3D Modeling for a profession and making money, a gamer card will work 90% of the time.
The price difference is basically, it is a different market, a much more expensive market, thus they can have a higher markup. Also having to do all the compatibility with software vendors is not cheap. The actually hardware is sometimes 100% identically depending on which card you are talking about. It is all in the software.