You need to specify your riding style and environment (rural, city, lots of lights, street lighting, do you need the light to see by, etc) and how much runtime you need.
I have a Cygolite Metro 550 which is nice for what it is. Bright enough for me to see by in rural areas, bright enough flash to work as a daylight visibility aid.
on the back, for < $100, I guess I'd go with a Axiom Pulse 60 for nighttime. Nothing < $100 is what I'd call daylight visible. For a bit more I'd try an Orfos flare. Personally the minimum I'd go for on the taillight would be a Dinotte Quad Red. I haven't tried the Orfos yet, it just looks good and is closer to your budget.
I spent more on my taillight than my headlight and I'd do it again.
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Work: the 8 hours that separates bike rides.