Why a putting green costs more
A putting green is a precision surface, not a flat lawn, and that is where the extra cost comes from. The base has to be graded and contoured so the ball rolls true, cups are set and drained, and the turf itself is a specialized short-pile putting product that costs more than lawn turf.
The result is a higher per-square-foot installed price than a standard lawn. The calculator has a putting-green preset that applies the putting band automatically, so you can see a realistic range for a green of your size.