This thread was useful for me to help decide:
http://www.bikeforums.net/showthread...era-for-biking

There is lots of choices out there right now. Many camera are below $50 and over $250. Things to consider is the choice of option and durability of the unit. Over the years, I had one 640x480 pen camera I used aim toward the back broke on me and another key remote that had the button popped out of the casing. Good thing is that the price is cheap enough where it really did not matter. Another thing to look into is the issue with water resistant of the unit. This is where I recommend the better quality camera.

To capture license plate, you will need a HD quality recording. My Contour does OK with it in the 1200 mode. However if the car passes me to quickly then it sometiime does not get a clear picture of the plate. I can set it to 60fps, but then I noticed that the plate number is not clear at a distant. I learned that by setting it to the 1900 mode at 30fps, plate numbers comes out pretty clear. The setback to that resolution is the usuage of the memory card. I find myself using 5 to 7 GB in that mode for a full commute video. My 16gb card is good for two days before I have to erase it. No problem until I have to load it to the computer where it take a while to read. A higher class card like a class 4 or higher may help.

The cost is major when considering having to spend over $200 or just $50<. I waited till Amazon had that Gold Box deal before I purchased my helmet cam. If cost is an issue, then get an affordable price HD from Ebay for now and see how you like it before spending more money. You can use that Ebay camera later on on something else if you upgrade when you find a good deal on the higher cost camera.