The best bang for your buck would be a lightweight LED flashlight powered by a 18650 battery. For less than $100, you could buy two flashlights (one for your bars, one for you helmet), a couple Two-Fish lockblock mounts, a battery charger and several 18650 batteries. If you don't mind spending a little more, the Lezyne Super Drive is a very bright, self-contained light and can be bought on-line for about $100. It also runs on 18650 batteries, and you could extend your run times significantly by buying a charger and some spare batteries.