I suggest you look up the values assigned to the land and the improvements (house) on the county tax assessors office. Use that ratio to determine what part of the cost basis was land and what part was the house. Then take 1/3 of that and that is a reasonable way to determine the cost basis for that part of the land.
For example, the tax assessor says the land is worth $90,000 and the "improvements" are worth $110,000 for a total value of $200,000 (ignore what the actual value is). The ratio of land to total value is $90,000 / $200,000 or 45%. So of the original cost basis of $65,667, the land was 45% or $29,550. One third of the cost basis of the land would have been $9,850.
