If you like to tinker, you can get yourself a CVS disposable video camera (30 bucks, as little as 15 bucks depending on coupons/sales) and modify it. You'll have to solder a usb cable to it, Then you download some free hack software and you're in business. (I'm assuming you have a computer).

I've never attached mine to my helmet, but I have attached it to my bike using twine and a some foam padding. It works well for thirty bucks. You can shoot 20 minutes of 320x240 29fps video.

If you google "cvs video camera hack" you'll get more than you wanted to know about it.