For inherited assets, the cost basis is the fair market value of the stock on the date of death. (If the estate chose the 6 month later valuation date, then the FMV on that date.) If that date fell on a weekday, you can use the average of the high and low stock price for that date. If it fell on a weekend or holiday, take the averages for the business day prior and the business day after and average those two. You can use https://www.praxair.com/investor-relations/stock-information-and-price to look up historical Praxair prices.
