These are high quality cameras, that have to work in large range of weather and lighting conditions. This cost might also include the software to OCR and store the details.
One of these companies does in fact have an iPhone app that police officers can use to query a national database, but it can't scan 1800 plates per minute as you cruise down a highway.
Here's an example large manufacturer of these: http://elsag.com/mobile.htm
Also note that they help local and state agencies obtain grant funding from federal sources, so I'm sure there's a bit of cost-cutting that could occur in a less bureaucratic and funded market. Also, these were like $30k 10 years ago, so prices are falling.
The cameras used for OCR on license plates need to have a fairly high resolution, fast shutter speed, and a number of other little additions which make it more expensive than an iPhone camera. Granted, $10,000 is really expensive for such a device, and the real cost for something like that is probably a few hundred dollars, but after you add in software and radio communication (probably a cell plan if I had to guess), you're looking at an expensive device.
Of course, we could build one ourselves for a fraction of that, but that's why we don't drive tow-trucks or license plate scanner cars for a living.