It's a fleet policy. Not really very different from what FedEx or UPS or any company that has a lot of vehicles on the road has.
There have been fleet policies out there for years.
I'm sure it's expensive, but everything is relative.
If UBER's numbers are to be believed, and they are doing 40,000 rides a day, each ride charging $1.50 rider fee, that's $60,000.00 per day. The insurance cost will be marginal compared to what they take in.
It's a innovative insurance ; so vital information insurer's has was the track record of uber drivers which is quite good ; as uber was already paying premium for the rider and third party , additional insurance is for the driver and car ; has to be between 100-150 dollar per car