Installation and maintanance costs are cheaper for grid then rooftop due to economies of scale. But you are also missing the wind complement advantage the grid has. The wind blows when the sun is not shinning. So a lot less backup storage is required for a renewable grid system then for an offgrid system based off solar.Ten solar panels on 100 rooftops, no different from 1000 in a paddock. Only the installation costs.
Often (Usually ) rooftop can use existing infrastructure, where a solar farm may need something built.