Convert that into tonnes, and then calculate how much biosphere could be supported with that volatile weight. And once it is gone, it is gone. It’s not a renewable resource. You’d be trading away billions of years of future Martian biosphere.

Oxygen and nitrogen are hardly rare substances in the solar system, and probably not even on Mars. There are near infinite number of "tonnes" on Mars.

Besides, if the alternative is to leave Mars dead forever, I'd rather let it live for a billion years.

But maybe I don't get what your alternative is, if any?

Nitrogen and hydrogen are basically only really available via volatiles. They aren’t components of common minerals in the rocks.

The alternative is to spin up a magnetic field with superconducting magnets to protect that atmosphere from the solar wind.