.My light is based on a 12Volt 20Watt Halogen bulb with a dimmer and a NiMH battery. It's easily powerful enough for trail riding. I think you could build a basic version of it (no switch or dimmer and with a cheaper Lead-Acid battery) for well under £50.
I just wondered if anyone else had done, or was thinking of doing something similar. Also how this compares with what you can buy 'off the shelf' these days.