Uhm. I think there is some misunderstanding here. These wildlife crossings are more than just a plank going over the road. They are basically a bridge with a small patch of forest on them (or whatever vegetation matches the surrounding area).
> To encourage use by wildlife, the bridge will have lush but drought-tolerant vegetation with matte materials to deflect bright headlights and insulation to quiet the roar of cars. Fencing at each end will help funnel them onto the crossing.
I don’t know what is the fair price of a 165-foot-wide (50 m) and 200-foot-long (61 m) overpass holding up a small forest, but I know that the additional weight of the animals crossing is not a significant factor in it at all.
Now obviously if we could convince wildlife to use something much narrower and simpler built like a pedestrian bridge maybe we could drive down the costs, i’m not sure if that has been tested or not.
One issue with narrow passages is that predators may learn to take advantage of them, eventually driving other wildlife away.
It's measured in billions.