User blog comment:KnocknickKnock/9 gates + master of the celestial realm/@comment-24066070-20131208182752/@comment-24264679-20131214020738

For some of the zodiac spirits Hiro mashima did provide information on how much magical power it consumes to summon. It was in Fairy Tail Volume 4 bonus content. You can find it on this exact wiki too. Not all the spirits are listed I believe though. Just reread your comment and you say it comes from the game but Fairy Tail Wiki says it comes from Volume 4 bonus content. I go with the volume 4 bonus since all the spirits aren't even listed just the ones from volume 4.

The summon powers are: Aquarius:100 Taurus:52 Cancer:51 Virgo:48 Plue:1 Horologium:16

With this being said Lucy pre time skip could open 2 or more keys if she desired. Thus meaning Lucy and Angel magical reserves pre time skip could have been roughly equal for all we know. As far as Urano Metria is concerned Lucy wasn't out of magical power she was just running a little low but she had enough to cast UM. She also had a little more left over after performing the spell to since Virgo can out and later on it was explained as Lucy gets stronger her spirits can summon themselves. Like Nonickneeded said during the GMG it wasn't as strong before because she had only practiced for a short period of time less than a day. It was even implied that she only practiced when she was in the shower.