Seems like you're missing the point. If a set of parts costs you X, and you can sell it as one product for 2X and as a different product for 3X, which one yields better profits?
We're not talking past each other. A Cadillac and a Chevy are much closer to parity than a Model 3 and a Model S/X (considering both parts and manufacturing techniques). I don't know what the Model 3 margins are yet, but I'm confident they're going to be above 15%.
Ask Bob Lutz whether margins are greater on a Chevy Camaro or a Cadillac ATS (exact same platform, one sells for about $10k more than the other).