Derek Lyons wrote:

It's highly unlikely it would have come down much even with one of the
original designs - which would have shared many of the same
maintenance and operating costs, and would have been even more more
expensive to research, develop, and build.

What's more, if the development cost had been much higher, NASA could
not have maintained even the pretense that the shuttle would have
had a positive return on investment, even if it *had* reduced
launch costs. The required flight rate to 'earn back' the development
cost would have been too obviously beyond what future congresses
would have funded.

Paul