since you both meet the rules for the maximum exemption, the first $500,000 of gain is excludable (not taxed at least for federal purposes). so if you sold net for not more than that (gross sales price less selling expenses), you will not be taxed on any gain. basis in this case is irrelevant so do the best you can to estimate but don't worry or spend a lot of time trying to get an "accurate" number. .
