User blog comment:KnocknickKnock/9 gates + master of the celestial realm/@comment-24066070-20131208182752/@comment-7418318-20131213093439

And the part about the cost of the different Zodiacs is just speculation. In no place in the series is it ever explained how much magic power each Zodiac consumes in comparison to the others. It's only said that golden keys use up more energy than silver keys and that Plue uses only a minimal amount. And the magic cost from the games are just for facilitating game-play and come from the game-designer, rather than Hiro Mashima and thus proof nothing.

Lucy didn't even have enough power left to keep standing. Where the energy for Urano Metria came from isn't really known, but I seriously doubt it was just from her own depleted reserves (my theory on it is, that the spell channels energy from the Celestial Spirit Realm).

And to clarify about Caelum: I'm not saying it's bad, just that it might not be quite as great as it appeared at first glance.