The mortgage payoff is not part of it. It is purchase price, improvements, sales price, closing costs and depreciation.
The first step is to separate out the land. If the land has not gone up or down in value since your bought the property, use the same percentage as you did when you first entered it. The sales expenses are prorated by the building/land proration. For example If the entire sales amount is $400k and the land value of that is $100k, the 75% of the closing costs are allocated to the building and 25% to the land.
