The mishna in ketubot talks about the enactment of the bet din shel kohanim (the court who oversees all kehuna-related activities) to increase the ketubah amount for a bat kohen marrying a kohen from the standard 200 zuz to 400 zuz – what does this mean?
The Mishna writes how the bet din shel kohanim would require the amount of 400 zuz for a בתולה, interpreted as the marriage of a bat kohen to a kohen (Rashi, Tosfot Yom Tov, Sedei Yehoshua) – this a two-fold increase of the standard 200 zuz amount. This entitlement is discussed in talmud bavli and endorsed by the rishonim (Nimueki Yosef, R”Y of Luniel, Rabbeinu Asher) most notably tosafot who deem’s it מנהג טוב מאד הוא – a very good minhag.
A different understanding of the mishna is given by Rabbi Yossi bar Bun (talmud yerushalmi); the increased amount was enacted by the bet din shel kohanim as a penalty. The penalty being in both instances, one where the bat kohen married a non kohen and in the instance where the bat yisroel who married a kohen, the amount of 400 zuz was listed in their ktubah amount. Reason being as chazal wanted each to cleave to their kind and the exaggerated ketubah would serve as a deterrence for intermarriage.