I think our experience with the shuttle has shown that the calculation is more complicated than it first appears. A re-useable spacecraft makes sense on paper, it doesn't have to be rebuilt from scratch each time you use it. On the other hand, it has to be designed to be used many times and to last a long time, and it requires a large maintenance infrastructure. A one-shot vehicle only has to work once, so the design issues are not as difficult, and you do not have to design in the inevitable maintenance features. Perhaps a generic modular concept is the best compromise, but that will work against overall efficiency (you won't be custom designing for the application).