As of August 2015 a sheet of duct board, or galvanized steel joist lining as the Lowe's website refers to it, is $5.96 for a sheet that is 16 by 36 inches. The website for Lowe's does not offer any other products or pricing for duct board.
Duct board is also readily available at many other home improvement stores, with pricing information included on their websites. Home Depot offers nearly a dozen duct board products on its website. Specialty duct board products, such as register, terminal, and return air boxes, range in price from $12 to $45 on Home Depot's website.