In general, the cost or other basis is the cost of the property plus purchase commissions and improvements. If you inherited the property, the cost basis is usually the fair market value of the property on the date of the decedent's death, but special rules can apply. If you received the property as a gift, the cost basis is usually the same cost basis of the person who gave you the gift.
For example, if your mother gives you 100 shares of ABC stock, your cost basis would probably be whatever your mother paid to purchase the 100 shares of ABC stock. Special rules apply if the value of the property is less than the donor's cost basis.
What if I don't know the cost basis of the stocks sold?