The most common materials used for wall sheathing are either plywood or OSB (oriented strand board). It is easy to see the cost difference for one sheet, but sometimes you need to know the overall cost difference. And in general, OSB tends to be cheaper than plywood.
40' 6" * 18' 4" = 40.5 * 18.33 = 742.5 square feet A 4' x 8' sheet = 32 square feet 742.5 ÷ 32 = 23.2
It will take 23.2 sheets to cover this section of the roof. And this is the number of sheets that determines the cost. There is no allowance for waste included.