All depends on the design of the deck, area, materials and contractors some contractors are affordable some vary pricing but can run from $12 to $18 square foot in VA for treated wood and up $35 for composite
There are alot of factors involed type and quality of materials being used were you live area of nation. If your looking for a rough estimate on the project you can google certain phrase and your city and get a idea of any type of project that cost there per sq ft even will give you a breakdown on the average labor cost in sq ft priceing guildlines. Of course this is just a rough idea of cost but you will know therange of what it cost and if the contractor coming to give you a estimate is giving a fair marketg price