Amazon offers the iON Air Pro Lite Helmet Camera, model no. 1011L, for $49 with free shipping. That's the lowest total price we could find by $126. It features 1080p and 720p resolution filming, battery life of up to 2-1/2 hours, and is waterproof to 30 feet.

Note that it's temporarily unavailable, but can still be ordered at this price for delivery when available.