At first i would calculate how huge the budget is. Commercial bicycle brakelights in mass production are 40-120€, so i guess 200$ for material and 200h of work could be possible.
Depending on the grade of existent electronic/soldering and IT/programming skills.
Role models can be theese:
Cateye Rapid X2 Kinetic (TL-LD710-K)
Litecco Twinfire
Lupine Rotlicht
See.Sense ICON+