[quote=@Abillioncats]
[@Lugubrious] Say since we're on the topic of upgrades. If I wanted to say upgrade Emerald's cannon so that firing it only takes a quarter of her aura instead of all of it, perhaps at the cost of firepower, how much would that cost?
[/quote]

It depends on how much that would improve her.  If this is more of a change for efficiency than a huge bonus, it could be a minor upgrade, but otherwise it's definitely a medium at least.