Bike lights :)
Ok, you have to be specific for what you're after. "low-cost" could mean getting something that's not going to work well for your needs.
- What kind of run times do you need?
- Do you have a preference for mounting options?
- Do you want a removable battery?
- How bright do you need it?
- Do you need any fancy features, like a percentage readout for the battery?
- Are you using it for a commute? Offroad? Bike touring?