They can cost anywhere between $3,900 and $4,500 for a 12-by-18 deck, though you can get individual deck boards for $20 to $50 per square foot. Composite decks may cost less initially, but they have a lot of maintenance over the lifetime of the deck, especially if you install pressure-treated lumber.hen properly maintained, this type of deck should last for decades. The cost for treated lumber ranges from $8 to $20 per square foot, for an estimated total replacement cost of $2,560 to $16,000 for a 16-by-20-foot deck.