I found a nice buy at Harbor Freight Tools, where they have a bike headlight for $7.99 (follow the link). I've used it quite a few times. It has 3 bright white LEDs and runs off of 3 AAA batteries. I'm not sure about the "lumens" rating, but it sure lights up the path in front of you quite well. Mounting and removing the light is simple, since it uses a thumbscrew to tighten its mount on the handlebars. It also removes easily from the mount to be used as a quite capable flashlight. In the "continuous" mode, its reported that the batteries last 70 hours, and double that in the blinking mode. I can't attest to the battery life (I'm only on my first set of batteries), but I do know that so far (after several hours of use) it hasn't shown any signs of dimming. We plan to return to purchase another one for my wife, since the Harbor Freight light is brighter than the one from the LBS that cost us $10 more.

They also have a tail light, also running off of 3 AAA batteries, that has a steady state or 3 different flash patterns ... for $5.99.