Why? Those servers still have to pay the same price for components plus a markup for the service. In theory you can serve more gamers per GPU, but these GPUs have to be physically located in your city to have a usable latency, and that means you'll have issues with peak utilization being most users gaming at the same time of day.
I just don't see the cost savings of sharing a GPU overcoming the extra expense + profit such a service would need.