I’m expecting them to give me $25k tax free to devote space in my home for the purpose of working during my work hours. I retain ownership of the property itself, but not the equipment; I would also reserve the right to use the space as I please outside of work hours, because it’s located in my home.

Those are my WFH terms. Companies can take those or provide me an office.

Fine, just say that figure; no one cares how you arrived at the figure (multiplying square feet times purchase price, nor what eggs cost at your local grocer).

The point was that real estate ain’t cheap, and I’m certainly not lending it to my gigacorp employer for free.

Of course it's not. You're free to set whatever price you want, but if you set that price and then try to explain it as "this is what it cost me to buy it", that second part undermines your first part.

Your employer, to the extent they want to agree to this, is renting space from you and you're citing and setting the price to buy it forever. That undermines. Just name your price and see if they take it or counter.