The sales price. Every penny of it. For the FMV, you may be able to check that at the county courthouse. When real property is transferred, it's done so with two valuations. The first one is the tax value, which is determined by the county property appraiser. They use that value to determine the property taxes.
The second one is the FMV (Fair Market Value) and that is generally done by a qualified and licensed property appraiser - not the county appraiser since they only establish a value for determining property taxes. In the case of raw land, the two values are usually close enough that they'll only list the county's appraised tax value.
Generally, the county's valuation is significantly lower than the FMV. If this is your case, then using that value would result in a higher taxable gain on your sale. What you can do is contact a real estate agent and ask for a CMV (Comparative Market Value) for land in your local area back dated to when your grandmother passed. Then you can use the CMV to establish your own FMV. You can expect to pay maybe $100 give or take for the CMV, but it could be worth it tax-wise. Only you can make that call. You'll also want to retain all paperwork associated with this for at least 7 years, in case you are ever questioned on this by the IRS. If you do it at a price the IRS consider's "fair", there's no reason you'd need it. But then, you can never be over prepared with the IRS.
