Gemara

(Rav Yehudah): Whenever the Torah specifies a quantity of "Kesef", it refers to coins of Tzuri (pure silver). Whenever Chachamim mentioned Kesef, they refer to standard coins (which are one eighth silver).

Question: We already learn this from a Mishnah!

(Mishnah): The following are in Shekel ha'Kodesh, which is one twenty-fifth of a Maneh Tzuri: five Sela'im for Pidyon ha'Ben, 30 Shekalim for a slave gored by an animal, 50 Shekalim for raping or enticing a virgin Na'arah, and 100 Shekalim paid by one who falsely claims that his Kalah was not a virgin.

Kesuvos 10a (Beraisa): "Kesef Yishkol k'Mohar ha'Besulos" teaches that the Mohar (Kesuvah) of a Besulah should be like the fine for a seducer (50 Shekalim). This was Chachamim's source in the Torah for the Kesuvah.

12a (Rav Ashi): First, they enacted a Kesuvah of 400 for a virgin Kohenes and 100 for a widow. Kohanos widows were disgraced, so they enacted that their Kesuvah will be 200. Men stopped marrying them, since for the same amount one can marry a virgin Yisraelis. They reverted to the initial enactment.

39b (Beraisa): If a rapist dies, the fine he paid is in place of a Kesuvah.

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, he pays coins of Kaputkiya (if he married her there).

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

(Rabah): Chachamim are lenient to allow paying smaller coins, for they hold that the Kesuvah is mid'Rabanan. R. Shimon ben Gamliel holds that it is mid'Oraisa.

(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).

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

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.)

Ran (DH mi'Kulei): Since R. Shimon holds that Kesuvah is mid'Oraisa, also in the Reisha he should require Tzuri coins! Rather, even though "k'Mohar ha'Besulos" obligates a Kesuvah mid'Oraisa, it need not equal the fine of a seducer. Rather, it is whatever they agree upon. Therefore, it is paid in coins of where he obligated himself. Chachamim fixed it to equal the fine.

Gra (EH 66:22(17),23): In the Yerushalmi, Tana'im argue about whether it is paid with standard or Tzuri coins. Tosfos (67a DH Omar) says that the Tana'im argue only about coins of Eretz Yisrael or Kaputkiya, but all agree that they are Tzuri. A Beraisa (39b) says that the fine is in place of a Kesuvah. This implies that also the Kesuvah is paid with Tzuri coins.

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 the Kesuvah is mid'Rabanan. We collect 200 Zuz of standard coins, which is 25 Zuz of Tzuri coins. Some say that Kesuvah is mid'Rabanan, but they enacted to pay (Tzuri) money of the Torah, lest divorce be light in a man's eyes. 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.

Teshuvas ha'Rif (176, or Bilgori 78): If the Kesuvah specifies a known coin, it is paid in that coin. If not, it is paid in the smaller coin, like the Mishnah says.

Rambam (Hilchos Ishus 10:7): One must write for a Besulah no less than 200 Dinarim (Zuzim), and for a non-virgin no less than 100 Dinarim. Chachamim enacted Kesuvah, lest it be light in a man's eyes to divorce his wife.

Rambam (8): These Dinarim are from the currency of that time, i.e. seven eighths copper and one eighth silver. A Sela (four Zuz) was worth half a Zuz of silver. Therefore, the 200 and 100 Dinarim for a Besulah and non-virgin equal 25 and 12 and a half Dinarim of (pure) silver. Each Dinar is the weight of 96 barley seeds.

Magid Mishneh: Nowadays, we write 'on his own, he added 200 Dinarim of pure silver, eight Zuzim per Dinar.' The Rashba (Teshuvah 1011) agrees.

Question (Drishah EH 66:14*): Why is a standard Zuz only an eighth of a silver Zuz? This ignores the value of the copper!

Answer #1 (SMA CM 420:60): The amount of copper in standard coins varies over time and from place to place. (Regarding paying for embarrassment,) the Shulchan Aruch discusses only the amount of silver. (Indeed, one must also pay the value of seven times as much copper.)

Nachalas Shiv'ah (brought in Chavos Ya'ir 1): Indeed, we write a Kesuvah of 78 gold pieces for a Besulah in order to include the value of the copper. The copper in 50 standard Sela'im is worth one and a quarter gold pieces.

Answers #2,3 (Chavos Ya'ir 1): Perhaps standard and Tzuri Sela'im do not weigh the same, or a standard coin is slightly less than an eighth silver. Gold Dinarim weighed more than silver Dinarim (Tosfos Bava Metzi'a 44b DH Echad). Perhaps Tzuri coins weighed slightly more than standard coins! The Agudah says that we are concerned only for the amount of silver in the coins.

Answer #4 (Gra YD 305:4): The copper is not worth the cost of melting the coin and separating the silver.

Poskim

Shulchan Aruch (EH 66:6): The Shi'ur Kesuvah is 200 for a Besulah, and 100 for a non-virgin, in standard coins. These are 37.5 and 18.75 Drahams of silver.

Rema: See YD 294 and 305. The above opinion is like the Poskim who say that the 200 Zuzim of the Kesuvah are mid'Rabanan, (25 Tzuri Zuz, i.e.) 6.25 Sela'im. Others say that the Shi'ur Kesuvah is in mid'Oraisa Zuzim (eight times as much), and it is 10 gold pieces for a Besulah and five for a non-virgin.

Chelkas Mechokek (22,23): In YD 305, it says that five Sela'im (20 Tzuri Zuz) of Pidyon ha'Ben are two Polish gold pieces. If so, Kesuvas Besulah (25 Zuz) is 2.5 gold pieces. Why didn't the Rema bring the latter opinion (that requires 10 gold pieces, i.e. four times as much) for Pidyon ha'Ben? The Agudah says that nowadays we write a smaller Kesuvah based on R. Tam, who says that 200 Zuz for a Besulah are 10 gold pieces. This is mid'Oraisa (pure silver); Pidyon ha'Ben is a tenth of this. The Rema did not bring the latter opinion, which is lenient, for Pidyon ha'Ben, which is mid'Oraisa. The coinage changes over time and from place to place. A Besulah's Kesuvah is silver the weight of 2400 barley seeds (according to the opinion that it is mid'Rabanan).

Beis Shmuel (13,14): The Rema brings the lenient opinion only according to the opinion that Kesuvah is mid'Rabanan (but we pay Zuzim mid'Oraisa). It is 'eight times more', i.e. 50 Sela'im, as opposed to 6.25. It is not eight times the Kesuvah mid'Rabanan based on the Shi'ur given in Yoreh De'ah.

Gra (YD 305:4): Earlier, Chachamim fixed the Kesuvah to be 200 silver Zekukim, which was 400 gold pieces, or in some places 500. Later, Zekukim were made partially from copper. The same amount of silver was worth over twice as much gold as before, but they kept the Kesuvah to be 400 or 500 gold coins. People married, trusting that the Kesuvah is like it always was.

Igros Moshe (EH 4:92): A gold coin equaled a Zuz of the Gemara. They enacted that every Besulah receive like a Kohenes (400 Zuz). Every widow gets half as much; men will not cease to marry widows. Women collected a Kesuvah of 400 gold coins (of Lita, or 500 of Poland), but the Kesuvah said '200 (silver) Zekukim' to teach that if gold (and hence divorce) will cheapen, we will again collect Zekukim. Nowadays the Shi'ur of gold is about 40$. This would not deter even a pauper from divorcing his wife! Also, one cannot demand gold or silver from the government treasury in exchange for our paper money; it is not Shavah Kesef. The Chasan is obligated to make the wedding feast. The custom is that the Kalah's family makes it. It is a proper compromise to add to the Kesuvah the cost of a normal wedding feast for people of their means. Nowadays, one may not divorce against her will, so he must give her a Kesuvah that she agrees to. Shi'ur Kesuvah pertains to a widow or a divorcee who demands what letter of the law dictates.

Igros Moshe (YD 1:189 DH v'Lachen): We fixed the amount in gold, because gold became the primary currency.