I have a Magicshine, and for $85 it puts out as much light as I possibly want for riding at speed even on bad roads. They claim 900 lumens, a couple of people have measured them at closer to 400, but even at 400 lumens, it's a heck of a deal at $85.

100 lumens will get you around, but you will have to take it easy; no riding at 25 MPH. 200 lumens is about the minimum that I personally would consider riding at full speed, and I'm not super comfortabel there.