I've been using a BM Dymotec 6 sidewall dyno for years and am really happy with it. I use it with a Spanniga headlight and a BM Toplight line Brake Plus and have all of $120 in the system. What is really nice, is the convenience of switching it from one bike to another on a seasonal basis despite different sized wheels. Look through Peter White's site, there are better headlights that are compatible and fall in your budget.

Marc