That does assume that the hardware can be reused without significant refurbishing. The extensive maintenance needed due to damage incurred each flight was one reason the reusable shuttle ended up with a much higher per-launch cost than the expendable Soyuz. One can surely do much better than the shuttle, but cheap expendable systems are not that easy to beat with a reusable vehicle.
The Shuttle was a political football that had extreme requirements placed on it that were never used as part of its subsequently-shelved military mission. The Shuttle also pushed the envelope extremely hard. Finally, its funding was cut severely during development. All of this made it much less reusable than it might have been.
The SpaceX project suffers from none of these problems. A relatively conventional rocket with medium-performance engines, no crazy DoD requirements, and a sane budgeting process should do a whole lot better when it comes down to being actually reusable.
Re-using without massive refurbish costs is their ultimate goal, hence the assumption. If things go according to their plan, they are going to have turn-around of single-digit hours. Avoiding massive refurb is one of the reasons why they are not keen on dropping their rockets into salt-water for several hours. The Shuttle SRBs were really only nominally reusable. SpaceX is going for actually reusable.
A Falcon 9 lower stage also isn't a spaceplane covered in very expensive and fragile tiles, sitting next to a rocket. That helps.
Just using the first-stage without significant refurbishing will bring costs down like 50%. SpaceX has already tested their rocket engines for multiple (long) firings, but I don't know how much they refurbished them.
