Interestingly, to get the $99 student price you have to be in a class that requires using it. I got one for one of my classes this year. Even buying it at the university bookstore, I had to fill out a form with my student id, course number, etc to verify that I was in the class before I could buy it.
If you look it the component cost it is easy to see why though. ~$50 for the ADC in 750+ quantity, $12 for the DAC, plus the FPGA, FT232, case and cables, development costs... obviously it is heavily subsidized by both Analog and Xilinx. I'm guessing the requirements for student pricing are because they want to push schools to make it part of their curriculum. Random students buying it for their own use don't give the companies nearly as much benefit for their money as a school actually including it in classes.
As far as the USB interface, I'm guessing they are using the FT232 chip in parallel mode as mentioned above. I have used it with 2 scope channels and 2 function generator channels running at the same time and it still updates very fast. The scope display responds pretty much as quickly as any of the real scopes I've used.