I think the big decision for a bike lite under $100 is whether you want to go with a flashlight or a bike-specific light.

The flashlights have the advantage of light weight, simplicity, flexibility, easy to get batteries, etc. The true bike lights have the advantage of runtime, maybe more bright.

Decide which type of system you like & go from there. I've used both and each have theri ups and downs..
Cheers