LightInTheBox offers this Cree XM-L T6 LED Front Bike Light for $5.99. Coupon code "SO5345026" cuts that to $4.99. With free shipping, that's $11 off and the lowest total price we could find. It features a 500-lumen output and waterproof body.
Note: Uncheck "Add Shipping Insurance to your order" during checkout to remove the 49-cent fee.