As I explained in my last post, the cost curve in the Treasury’s model of the RSPT is based on a fantasy. The situation is even worse when we turn to the other side of the model, the demand curve: it is based not merely on a fantasy, but an outright fallacy.
Legions of economists believe that the demand curve for a competitive firm is horizontal, but their belief is based on the most fundamental of mathematical errors: confusing a very small amount (an infinitesimal) with zero. The Treasury repeats this fallacy—without knowing it is one—in its explanation of why a royalty reduces output levels but the RSPT won’t.