It's all dependent on context for a period race car and race engine 50mm Mikuni's are rare enough that they demand a Premium price, they always have relative to other 44's etc.. Given the recent rise in values of other S30Z cars and parts it's no surprise that the 50's have shot up.
Even 44's are now fetching $3-4K AUD sometimes more..
As for how they all perform, I'd love to see back to back tests between Weber, Mikuni, SK, Solex, SUs, Dellorto's on a dyno and on the same engine being road driven to see how they all compare in terms of progression and drivability / stumbling / AFRs and of course power band etc.. but perhaps that's a dream? From what I've read most people rave about the Mikuni's.