Gemara

10a (Rav Nachman): Chachamim enacted that a virgin gets a Kesuvah of 200 Zuz, and a widow gets a Kesuvah of 100.

Contradiction: In one Beraisa, R. Shimon ben Gamliel says that the Kesuvah is mid'Oraisa. In another Beraisa, he says that it is mid'Rabanan!

Answer: We must switch the opinions in the latter Beraisa. Alternatively, R. Shimon ben Gamliel teaches that the Kesuvah of a widow is only mid'Rabanan.

12a (Rav Ashi): Regarding Kohanos, first it was enacted to collect 400 for a virgin and 100 for a widow. When Chachamim saw that this caused Kohanos widows to be disgraced, they enacted that their Kesuvah should be 200. Men stopped marrying them, since for the same amount one can marry a virgin Yisraelis.

51a (Mishnah): If a man did not write a Kesuvah to his wife, it is an enactment of Beis Din that a virgin collects 200 and a non-virgin collects 100;

110b (Mishnah): A man can pay the Kesuvah in coins of Eretz Yisrael (which are smaller than those of Kaputkiya) if he married or divorced her in Eretz Yisrael;

R. Shimon ben Gamliel says, if he married her in Kaputkiya and divorced her in Eretz Yisrael, he pays coins of Kaputkiya.

If he married and divorced her in Kaputkiya, he pays coins of Kaputkiya.

(Rabah): Chachamim hold that Kesuvah is mid'Rabanan, therefore, they are lenient to allow paying smaller coins. R. Shimon ben Gamliel holds that Kesuvah is mid'Oraisa. (Therefore, he pays with coins from the place he married her.)

(Beraisa): A loan document is paid with coins of the place it was written. If no place is written, it is paid in coins of the place of collection. If it says only 'Kesef', the borrower pays whatever he wants. This does not apply to a Kesuvah.

(Rav Mesharshiya): The Reisha (it is paid in the coins of the place it was written) does not apply to a Kesuvah. This is unlike R. Shimon ben Gamliel. (One may pay smaller coins, for Kesuvah is mid'Rabanan.)

Yevamos 89a: Kesuvah was enacted lest it be light in a man's eyes to divorce his wife.

Rishonim

Rif and Rosh (65b and 13:20): The Tana of the Beraisa holds that Kesuvah is mid'Rabanan. This is the Halachah. (Rosh - R. Tam disagrees; see below.)

Rosh (1:19): Why is the custom in Ashkenaz to write in the Kesuvah '200 Zuz that befit you mid'Oraisa'? R. Tam says that we rely on R. Shimon ben Gamliel. The Halachah always follows him in the Mishnah. R. Chananel and all the Ge'onim rule like Rav Nachman, who say that Kesuvah is mid'Rabanan. We collect 200 Zuz of standard coins, which is worth an eighth of Tzuri (pure silver) coins. Some say that Kesuvah is mid'Rabanan, but they enacted to pay (Tzuri) money of the Torah, lest it be light in a man's eyes to divorce his wife. The custom is to write '200 Zuz that befit you mid'Oraisa', lest one err to think that it is standard coins. This is reasonable. One who writes '200 Zuz of silver that befit you' does not lose.

Bach (EH 66 DH Kosav): According to the Rosh, Kesuvah is mid'Rabanan, but it was enacted with Tzuri coins. Even if one wrote only '(200 Zuz) that befit you', she collects Tzuri coins. According to R. Tam, she gets standard coins, because the bearer of a document has the lower hand. In a place where they do not write a Kesuvah, she collects from Beinonis (medium quality land) if Kesuvah is mid'Oraisa, and from low quality land if Kesuvah is mid'Rabanan.

Rebuttal (Beis Shmuel 66:14): Tosfos (10a DH Ho'il) says that even though R. Meir holds that Kesuvah is mid'Oraisa, this is not why he holds that Kesuvah is collected from Beinonis. Rather, it is to encourage marriage.

Rambam (Hilchos Ishus 10:7): Chachamim enacted Kesuvah, lest it be light in a man's eyes to divorce his wife.

Rambam (9): If one reduces the Kesuvah of a virgin from 200, the Bi'ah is Zenus.

Lechem Mishneh: The Rambam cites the words of R. Meir. Even though the Rambam rules that Kesuvah is mid'Rabanan, Chachamim strengthened it more than Divrei Torah.

Rambam (Hilchos Edus 14:3): An adult can testify that when he was a minor, he saw that Plonis was married like the custom of Besulos. This is because most women are Besulos when they marry, and Kesuvah is mid'Rabanan.

Question (Lechem Mishneh): Just the contrary! Since the Torah does not obligate Kesuvah, we should require proper proof!

Answer (Or Some'ach): Kesuvah is an enactment lest it be light in a man's eyes to divorce his wife. It is proper to follow the majority to strengthen the enactment. If there is no Kol that she was married like a Besulah, this weakens the majority. We are lenient to accept testimony from one who was a child at the time, to enable us to follow the majority.

Poskim

Shulchan Aruch (EH 66:6): The Shi'ur Kesuvah is 200 for a Besulah, and 100 for a non-virgin. Both of these are standard coins.

Beis Yosef (DH uv'Hagahos and DH v'Hashta): The Mordechai (312) concluded that the Zuzim are Tzuri, both for a Besulah and for a widow. The Rif, Rambam Ran, Rabbeinu Meir and all the Ge'onim say that it is standard Zuzim. We follow them against R. Tam and the Rosh. If the Kesuvah says '200 Zuzim that befit you mid'Oraisa', it is not clear what is the law. Presumably, she gets standard coins, for the bearer of the document has the lower hand.

Rema: This is like the Poskim who say that Zuzim of the Kesuvah are mid'Rabanan, so the 200 Zuzim are (25 silver Zuz, which are) six and a quarter Sela'im. Others say that the Shi'urim are 10 and five gold pieces, respectively. This opinion holds that the Shi'ur is in mid'Oraisa Zuzim, which is eight times more than standard coins.

Gra (66:22(17)): The Ge'onim rule like the opinion that Kesuvah is mid'Rabanan, so it is paid with standard coins. They hold that this does not change throughout the generations. This requires investigation.

Rema (ibid.): Therefore we write 'that befit you mid'Oraisa'. Some say that we write just 'that befit you'. The custom is to write 'that befit you mid'Oraisa' for a Besulah, and 'that befit you' for a widow. This is where there is no custom. Where there is a custom, we follow it.

Drishah (14* DH ul'Fi): If we write 'mid'Oraisa' to show that the coins are Tzuri, we should write the same for a widow! Rather, it means that the Kesuvah is mid'Oraisa.

Rebuttal (Beis Shmuel 14): Chachamim did not enact pure silver for a widow, even according to R. Tam, who say that Kesuvas Besulah is mid'Oraisa. Only the Mordechai says that even a widow's Kesuvah is pure silver.

R. Akiva Eiger: Also Tosfos (67a DH Omar) says so. (He learns from 12a. At first, the Kesuvah of Kohanos was 400 for a Besulah and 100 for a widow. They made it 200 for a widow. Men stopped marrying Kohanos widows, for one can marry a Besulas Yisrael for the same amount. This implies that a widow's Kesuvah is half of a Besulah's.)

Beis Shmuel (14): Even according to our custom to collect for a widow half the Kesuvah of a Besulah, we cannot write 'that befit you mid'Oraisa', for Chachamim did not enact this.

Shulchan Aruch (69:6): If a husband stipulated to be exempt from one of his obligations, it is valid, except for her Onah or her Ikar Kesuvah.

Beis Shmuel (6): Even though most Poskim hold that Kesuvah is mid'Rabanan, Chachamim strengthened their enactments more than Torah law. A stipulation helps for food and clothing, which are mid'Oraisa.