I know cost-effectivness is often the deciding factor, but that's kinda sad, so: One deer hit a day, down 91% means ~330 deers saved per year. $15m spread over the working population (160m) of the US is less than $0.10 per person. So you're spending $0.10 to save 330 deer, which is a trade I would make.
You have to realize that it isn't just about saving deer. If you hit a deer at speed, it is going to do massive damage to your vehicle and potentially cause significant injury to the passengers, up to and including death. Overt fatalities are reportedly rare, like 0.1% or something, but the injury rate is closer to 5%.
The US does not have a shortage of deer. Mostly the opposite as their natural predators have been drastically reduced in numbers.