You should use the actual cost of the land that was sold, plus any expenses (real estate commissions, etc.) paid. If you only sold a portion of the land, for example - if you own 1 acre, and sell .33 acres, then you should allocate the original cost to the portion of land that was sold. You can use "any reasonable method" for the allocation - for example, 1/3 of the total cost; or if there are property tax statements that allocate the portions of land and provide their valuation, you can use that to pro-rate. As long as you can explain your method for allocation of the cost and it makes sense.
