Not to deter you from the DIY route, but if you happen to have an arsenal of rechargeable AA's on hand, three of
these plus a three-pack of
TwoFish lockblocks would be... lessee here... under $90.
At 800mA, those lights are probably producing 170 lumens at the emitter, and naturally not all of that makes it out the front, so take the ratings with a grain of salt. But that applies to most lights on the market. At any rate, it would be a lightweight, self-contained setup and you'd also be able to swap from bike to bike easily, and/or use one as a helmet light.
Regarding the 500 lumens: I've read a zillion threads and understand some think this is excessive while others see it as the minimum.
The beam pattern is very important, not just the lumen rating. My commuting setup currently consists of a L2D Q5 (rated 180 lumens) and a DiNotte 600L (rated ~600 lumens), which probably sounds like a lot, right? But it doesn't reach very far down the road. These lights spread their lumens out a lot, which is great for trail riding on a mountain bike, but not ideal for road riding where you want to see road hazards with plenty of time to react. So keep the beam pattern, and the
distribution of the lumens, in mind. Not just the lumen count.