There's the transportation and distribution cost on grid-scale that's currently low, but as the generation costs go down, the fixed cost of owning the land, building and maintaining power lines will occupy larger piece of the pie chart.

Residential has zero transportation cost, post-installation, and I think those solar roof tiles SolarCity pre-announced is an attempt to blend the solar installation costs into the roofing costs. "You are paying the roofing guy for labor and materials to build/replace the roof anyway, so why not pay our roofing guy to build/replace the roof, and get solar as an incredible bonus" is I think their marketing strategy with that.