Start with original acquisition costs....first step is to determine the allocation at that time of just the land, and just the timber. i.e. what would the land have been worth w/o any timber on it.  I live 15 miles from a paper mill, so I come across this all the time.  after you get the land and timber segregated costs wise, then you determine what % of the timber was sold...and that's your best gest estimate..