I bought one of the $39 "1600 lumen" magicshine clones from eBay. It works just fine. The downside is that the battery is NOT waterproof, so put it in something waterproof. I just put mine in my waterproof panniers. When I'm not riding with them, I just put it in a plastic bag with the opening towards the bottom; works fine..
I added a lens to improve the beam pattern.
The light is even cheaper now:
http://www.ebay.com/itm/1800Lumens-C...-/321056565729
Here's the lens:
http://www.ebay.com/itm/Wide-Angle-L...-/280752532003