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I'd say this is a downplayed version of Sheltered Aristocrat.
Sure, she might've been "cost conscious" before (or so she thought) despite her lavish lifestyle, but that was when she was living off Mummi and Daddy. Now that she's the one footing the bill, she's finding out just how expensive those designer dresses etc., really are. Whereas they seemed reasonably priced when her parents were paying for them.
Edited by MiinU

Also called, as I jokingly put it 'Tsukuyo discovers adulting/gets sticker shock'
Basically the character involved, while raised rich, is also money minded. She understands hard work and budgeting, she is not the sort of girl who thinks money is endless. And for reasons she and her twin sister are looking to leave their families and live on their own somewhere because both of their lives suck.
However she starts doing some budgeting for how much rent and food would cost and she's incredibly stricken by it.
Looking for a trope that covers this without having Tsukuyo lack a concept of money's values.