I got a 808#11 from China via Ebay - cost £23. Already had a micro SD card for it, but you might need to buy one - you can get 32gb ones for £15 now. I mount it to my helmet with gaffa tape. It records at 720p, quality is decent enough to read number plates and its OK in low light conditions. The mic picks up the wind, but its not as bad as other much more expensive cameras. The only real downsides are the 1hr battery life - you'd need to charge it for every journey, or get a USB battery, or take it to bits and replace the internal battery. Also, you need to remember to clear the storage yourself, but a 32GB card should be good for a weeks commutes I'd guess.
Heres a vid with some demos and explanation of how it works.https://www.youtube.com/watch?v=uj26JjSiazQ
i use one of them 808 cameras (stripped down) on my radio controlled plane
mine is a #11 version its great for the price an weighs nothing, i have replaced the lipo with a 1200mh battery which would last waaaaaay longer than the sd card