I too run a magic shine. Great light for the money. Easy to mount and unmount or move from bars to helmet. Helmet mount is a bit clunky. DealExtreme is a great source for flashlights. I also have an MTE SSC P7 that can be bar or helmet mounted and runs on one 18650 battery. You can easily carry an extra battery in your jersey pocket or saddle bag to achieve your three hour run time.

I also would suggest a bar mounted and and a helmet mounted light, so your $50 budget might have to be stretched a bit to achieve that.