In the greater Boston area, I think I've only seen Zipcar spots on private property (though I'm not sure what the ownership is of the MBTA parking spaces) and I would expect the rate to be somewhat negotiable, based on supply & demand for the neighborhood.
There are car sharing services in Seattle that operate much like the scooters and bikes (park them in any city space, pay just for when you use, etc). The city charges a flat rate per car per month. Like dockless bike share it ends up being way more useful.
