As you said, mods are being paid in intangibles. If you log in to a website several days a week to fulfill the responsibilites that come with your position, in exchange for compensation, for years at a time, that's called a job. you may not be an employee but at some point you are working.
I'm not saying we can assign a dollar value to this work, I'm saying we can ballpark an amount of value and we might as well communicate that in dollars. If you'd like we can measure it in millions of hours donated. The magnitude of these numbers alone should make this interesting.
Because reddit has no close competitors there is no market for mods, reddit has a monopoly. Since there is no market to tell us what the value of moddding is we have to guess. If there were a hundred reddits competing with each other for mods we could trust the level of compensation they receive. Glut of supply and only one buyer means compensation will be far under what it would be in a "fair market".