I'm not sure I get it right, but here's my impression.
One-time rockets impose tight conditions on all parts' lifespans and quality: they must live through the launch with five nines reliability, yet making them last any longer is a waste of resources. Putting backups is a waste as well.
The reuse, on the other hand, means that (1) long lasting parts are not a waste and (2) backups are not a waste. This means that longer lasting, less reliable parts (i.e. closer to civil manufacturing, think commercial aircraft) can be used which in turn means much simpler production and QA. And *that* will drive the price tag down (eventually), not saving half the mission cost at half the mission cost.
*If* my assumptions are correct, *then* we're going to see a slight increase in engines number/power, and a series of successful launches/landings *despite* failing engines.