If the target is < $100, you might barely meet that and still get something waterproof but not if you want high definition. You won't be able to reliably read licence plate numbers without HD. You can slide in under $100 and get HD if you don't need waterproof.
The 808 #16 cameras with an external power source provide decent 720p video and you can get both for < $50. I ran with that for a month or two until I decided to pick up a Contour Roam. I like the Roam well enough. If they'd add loop recording it would be nearly perfect.
I also have a Crocolis Extreme Cam, which has a full set of controls and a screen. It's a little larger than the Contour Roam but it has a lot more features for less money, you can get it on eBay for $170 shipped (from China, so it will take 3 weeks or so to arrive). It comes with a ridiculous number of mounting options and a full set of cables. I like it pretty well if you're OK with a little more size and weight.
Here's a video from the Crocolis at 720p:
http://youtu.be/X1pWIpWtdxI
and 1080p (which gives a narrower field of view)
http://youtu.be/6SZy5MaUcEU
Here's the Contour Roam:
http://youtu.be/PbZIfsCzSME
Here's a 808 #16
http://youtu.be/GciAFakwdX4
Here's the 808 #16 mounted on my helmet with an external power source, 2xAA "emergency charger"
http://www.flickr.com/photos/1365953...in/photostream
Heres both the Contour Roam and the Crocolis cam on my helmet
http://www.flickr.com/photos/1365953...in/photostream