I bought a Drift x170 for about £80, as it isn't HD.
It attaches to my lid with a sticky bit that it comes with, however, I have also neatly used gorilla tape to add to the stickiness, as I didn't trust it. It has stayed on pretty well in all weathers and I have been over the ton and it has stayed on fine . . . . . .
I record in the highest quality possible (720) and it seems to use 1gb for every 30mins of recording and I use a 16gb SD card. The internal memory is shite, you WILL need an SD card.
With regard to batteries, it takes 2 AA's and it is supposed to record about 4 hours in the energy saving mode before replacing.
I put rechargeable ones in. Is a bit fiddly taking them in and out.
Annoyingly, no external mic, is quite big, attachments ain't that strong.
Good bits:- Is cheap as it's discontinued, not bad quality although not HD, pretty tough, water resist so it won't break in the rain (unless you drop it in a very deep puddle)
