It's an entirely different matter to take that into an airplane and use it in such a way as that my life depends on the accuracy of the information. Has it been extensively tested in actual airplanes in real-life flying? Does it work with and not interfere with other aircraft systems? Does it work with the software I already use and know well?
Yes, $100 < $500. Now, if I built my own airplane (something I've given some thought to doing), I might be a little more willing to experiment with it. But the majority of pilots are not hardware hackers. They're average, everyday people.
I will happily pay $500 for a tested piece of hardware that is guaranteed to provide accurate information and work in my airplane over something I homebrewed using instructions I found on the Internet. My safety is worth the extra $400.
http://flightaware.com/adsb/piaware/build