Here’s the documentation that shows it’s as simple as current fair market value less debt.
From the FAFSA:
*As of today, what is the net worth of your parents’ investments, including real estate?
Net worth means the current value, as of today, of investments, businesses, and/or investment farms, minus debts related to those same investments, businesses, and/or investment farms. When calculating net worth, use 0 for investments or properties with a negative value.*
Nothing about taking a guess (and that’s what it would be) about any possible costs that you might incur if you were to sell the rental property.
Similarly, the Profile only asks about “current market value” of real estate and “what you owe” on real estate. There are no questions that ask you to guess about how much you might pay in capital gains tax or broker commission if you were to sell the real estate.