The basis is the fair market value on the date of the death of the previous owner, even if no one bothered to clear the title. You can get probably a retroactive appraisal from a qualified appraiser in the area because they can use historical data on similar property. (Unimproved land is pretty easy to value, actually, as long as the zoning didn't change at some point.)
Divide the basis by the number of heirs. Divide the selling price by the same. That's your basis, your selling price, and your taxable gain.
