Cost of a Deck - Estimates and Prices Paid -…Average costs and comments from CostHelper's team of professional . Composite deck installation can cost $25-$70 or more a square foot (8,000 -$22,400 for 16'x20"). . cost for any type of deck include the height; shape; number of levels; railing ... This porch has a shed roof ,2ft knee wall, then windows to the ceiling .