The metrics are simple: it needs to take a punch and hit back, the cheaper the better. While Starfleet will never say no to engineering and scientific capability, what it really needs is something to dissuade the eruption of open hostility with neighbouring powers. Increased spending needs to be tactically justifiable, and the more ships the fleetyards can pump out of the resulting heavy cruiser design the better. For that reason the expectation is the ship should mass around two hundred thousand tons, which is what your cost and efficiency metrics are assuming.