I assume you mean the 1099S shows a selling price of $475000 NOT GAIN. you should not have to estimate anything. Your client should know how much his half of the purchase price was. If his profit was less than the $250000 exclusion (assuming he meets the requirements) then don't worry about closing cost, as it won't make any difference. Just fill out schedule D with His Basis, and 1/2 of the selling price.