Life Estate Sale 1099-S

My mother in law's home was a life estate sale with my husband and his brother listed as remainder interests . She needed to sell and move to assisted living. The 1099-S lists my husband's gross proceeds only, but the amount is less than the check issued to him at closing. I am recording the sale as an investment sale where the 1099-B is recorded. But how do I calculate the cost basis? Is it the cost of the home in built in1965 plus any home improvements that we have receipts for?