The other important factor is how many times you get to re-use the stage, because that tells you the saving per launch. If you get to re-use it once, then on each flight you save 37.5% of the launch cost. If you re-use it again twice (3 launches of the first stage) you save 50% of the launch cost per flight (each flight costs you 1/3 of the first stage cost). And so on. That's not factoring in refurb and re-fueling costs, of course.