Don't forget amortization of R&D costs
Sure, a fully reusable space booster could be (not a given*) be a lot cheaper when it comes to direct operating costs, but if the R&D is a lot higher (and your earlier post implied that the early Shuttle's R&D would've been about 4 times higher than the current design's), then the total cost per mission might well have been even higher than the current design's. A lot of people gloss over the higher R&D costs when arguing for fully reusable designs.
*Depends on how much effort is required to turn the vehicle around for its next mission. A fully reusable design should cost less than a partially reusable or non-reusable vehicle, but not necessarily depending on the technologies chosen.