How can I figure the cost of a stock held in a revocable living trust with co-trustees/grantors husband and wife in a non-community property state if the husband dies?

on the date of death take the high and the low of the stock add them together and divide by 2

The broker where the stock is held should do this for you automatically if you let them know he passed.  

I have Ameritrade and Ameriprise and they each used a different formula.