You can essentially already do this with a Raspberry Pi, PiCam, battery pack and opencv. The hardware will cost you under $100, and there are already various libraries that can process the license plates [0]. With a 3g stick you could even collect this data remotely.

Of course, performance won't be comparable to the commercial product here in question, but I'm certain that with some motivation you can get similar, if not better, results on better hardware.

[0] https://github.com/openalpr/openalpr