Electric vehicles cost about 1-3 cents/mile for the electricity. They also require so little service, I would assume the cost would be closer to 10-20 cents/mile for a self-driving electric vehicle.
but, sure, it would most likely be less.
At any rate, this eventual reduction in insurance is why I started by saying "Initially at least". With time, the cost of manufacturing self driving cars will also go down.
Electric vehicles would depreciate at a much slower rate than ICE vehicles, as the only components that wear would be moving parts (electric motor, fans for cabin comfort) and any degradation to the battery pack (Tesla has packs at 500K miles that still have 80% capacity).