I built one for <$100:
1. 4-D M@g host $20
2. 13xCBP1650 batteries $29
3. Kiu Switch w/bi-pin adapter $12
4. Aluminum reflector $20
5. Borofloat lens $5
6. 64623 bulb $4
7. IRF1503 FET & some caps/resistors (to switch the high-current) $2
8. Small plug to connect to my Triton charger
This does take some labor and knowledge since you have to solder the pack, install a connector to mate with your charger, and build up a small circuit with the FET.