If people want twice as many cars as there are, and they want twice as much toilet paper as there are rolls, do you expect rolls of toilet paper to cost as much as a car?

Supply and demand curves are (a) curves, not two points on a line (b) curves as a function of price. You can use them to pick a price, but not to come up with one from amounts of X ex nihilo. Even if we're sticking with the most primitive micro-economic theory here, you've discarded a whole dimension.