(($2M/launch) / (100 Mg / launch) = $20/kg
Now obviously those are aspirational numbers; Starship is too early in its development cycle for anyone to be making truly accurate cost projections. Furthermore the cost per launch to SpaceX isn't necessarily the same as what they'll actually be charging their customers. But even if Elon's guess is off by an order of magnitude, it'll still be way cheaper than "hundreds of dollars per pound", and it'll happen much sooner than "25 years".