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A prince falling for a peasant would be an interclass romance, which is the Uptown Girl trope. If it seems like a case of Uptown Girl at first and then is revealed that they are the same societal class, that would be a subversion.
The "suddenly revealed to be a princess" bit would be Really Royalty Reveal. The corresponding reveal for money is Secretly Wealthy. If the audience knew this all along, then it's an Internal Reveal.
If it's revealed that the poor girl is actually rich, and the prince makes clear that he doesn't care about the wealth, it's Not with Them for the Money.
Given how many tropes could apply here, it's possible I missed something. These relationship-type tropes are a never ending rabbit hole.
"I can now marry you because I turned out to be rich" is Suddenly Suitable Suitor.
Yeah, Suddenly Suitable Suitor is what I was looking for, thanks.
A story/fairytale about a prince who meets a peasant, they fall in love and he learns to look past class prejudices to be with her...then in the end it's revealed she's the lost princess from another kingdom. Convenient.
Genders might be inverted, too. Modern versions of the trope have the peasant/poor girl be revealed to be related to a rich family.