Well, I've experimented with GPS on several Nikon cameras. I would say that if you want to keep it under $100, you're probably looking at a GPS logger and post processing software to add the EXIF data. For real-time units, shoe or strap mount with a wired connection, Solmeta works well. Shenzhen EasyTag also does pretty well. But in either case, I think you're closer to $200.
I found that the wire connect units invariably caused problems. I caught the cable on stuff or killed the cable somehow. I have started using Foolography's Unleashed line. They have modules that work on all but the D800 and are working on that. The module plugs into the camera and gets its data from a GPS unit via bluetooth. They have the advantage of working with MTK chipset GPS units which seem to me faster to lock on than the SiRF units I've tried. You do have to have the module and the GPS unit (QSTARZ bt-Q1000X seems to be the best of those I have tried.) One downside of Unleashed with the D600 is that the rubber door on the GPS connector interferes with the module and needs to be removed. Of course, here too with the module and GPS unit, you're well over $100. BTW, Foolography is a German product, most of the others I've seen are Chinese.