Well, my first light was a DIY 20W halogen. Total cost was about $50. About $20 for the battery, $20 for the charger, and $10 for the light, the 2 or 3 plumbing parts to make the housing, and a few feet of wire.

It lit the road just fine, and I got mistaken for a motorcycle at night. I'd still be using it but I got tempted by the Trailtech HID for $120, and am still using that. When it dies I'll probably go back to an overvoltaged halogen, you just can't beat it for price/performance.