There's also the supply/demand aspect of it. Some electricity is cheaper to provide than others - the cheapest is the renewable or nuclear that's already built in the area, but when demand is high, the grid provider will source electricity from more expensive sources - coal, natural gas, or importing it from neighboring utilities. So, using some made-up numbers, if your existing cost for 100MW is $0.10/Wh, getting the next 100MW might cost $0.50/Wh, bumping the cost for everyone up to $0.30/Wh.