How to Calculate Mahr in Islam
Question

What is the minimum amount of mahr (Dowry) permitted by Shari’ah? I found in one book, the dowry of most of the wives and daughters of the Prophet (peace and blessings of Allah be upon him) was 480 Dirhams. What is the equivalent amount for the present time?
Praise be to Allah.Minimum amount of mahr in Islam
The minimum amount of mahr has been explained in the report in al-Sahih (no. 1425) narrated from Sahl ibn Sa’d al-Sa’idi, who said: “A woman came to the Messenger of Allah (peace and blessings of Allah be upon him) and said, ‘O Messenger of Allah, I have come to give myself (in marriage) to you.’ The Messenger of Allah (peace and blessings of Allah be upon him) looked at her and looked her up and down, then the Messenger of Allah (peace and blessings of Allah be upon him) lowered his head and paused. When the woman saw that he had not made a decision about her, she sat down. A man from among the Sahabah said, ‘O Messenger of Allah, if you are not interested in her, then marry her to me.’ He said, ‘Do you have anything?’ He said, ‘No, by Allah, O Messenger of Allah.’ He said, ‘Go to your people and see if you can find anything.’ So the man went, then he came back and said, ‘No, by Allah, I could not find anything.’ The Messenger of Allah (peace and blessings of Allah be upon him) said, ‘Look and see (if you can find anything), even if it is only an iron ring which you can give.’ So he went, then came back, and said,  ‘No, by Allah, O Messenger of Allah, not even an iron ring. But (I have) this izar (garment) of mine, she can have half of it.’ The Messenger of Allah (peace and blessings of Allah be upon him) said: ‘What can she do with your izar? If you are wearing it she will have nothing of it.’ The man sat down, then after a long time had passed, he got up (to leave). The Messenger of Allah (peace and blessings of Allah be upon him) saw him leaving and called him. When he came, he said, ‘What do you know of the Quran ?’ He said, ‘Soorah Such-and-Such and Soorah Such-and-Such.’ He said, ‘Do you know them by heart?’ He said, ‘Yes.’ He said, ‘Then go, you are married to her by what you know of the Quran.’” 
This hadith shows that it is permissible for the mahr to be a little or a lot of whatever is considered to be wealth, if both partners agree, because an iron ring is extremely little. This is the madhhab of al-Shafi’i and is also the view of the majority of scholars among the earlier and later generations. 
This was also the view of Rabi’ah, Abu’l-Zinad, Ibn Abi Dhib, Yahya ibn Sa’id, al-Layth ibn Sa’d, al-Thawri, al-Awza’i, Muslim ibn Khalid, Ibn Abi Layla, Dawud, the fuqaha of the scholars of hadith and Ibn Wahb among the companions of Malik. It was also the view of all the scholars of the Hijaz, Basrah, Kufa and Syria, and others, that whatever the couple agree upon is permissible (as a dowry), whether it is a lot or a little, such as a whip, a pair of shoes or an iron ring, etc. 
Mahr of the Mothers of the Believers
With regard to the question about the mahr of the Mothers of the Believers: 
Muslim narrated in his Sahih (no. 1426) that Abu Salamah ibn ‘Abd al-Rahman said: “I asked ‘Aishah, the wife of the Prophet (peace and blessings of Allah be upon him), what the mahr given by the Prophet (peace and blessings of Allah be upon him) was. She said: ‘The mahr that he gave to his wives was twelve uqiyah and a nashsh.’ He said, Do you know what a nashsh is? He said, she said it is half of an uqiyah. That was five hundred dirhams. This was the mahr given by the Prophet (peace and blessings of Allah be upon him) to his wives.” 
Ibn Khaldun said: 
“The consensus of the scholars from the beginning of Islam and the time of the Sahabah and the Taabi’in has been that the shar’i dirham is that of which ten coins weigh seven mithqals of gold. The uqiyah is forty dirhams of this type, and on this basis it is seven-tenths of a dinar… All of these amounts are agreed upon by scholarly consensus (ijma’).” (Al-Muqaddimah, p. 263) 
Based on this, the weight of a dirham in grams is 2.975 grams. 
So the mahr of the wives of the Prophet (peace and blessings of Allah be upon him) was 500 x 2.975 = 1487.5 grams of silver. 
The price of one gram of pure silver that has not been worked is approximately 1 riyal, so the mahr in riyals is approximately 1487.5 riyals. In dollars it is approximately $396.70 (this varies depending on the present price of silver).
For more, please see these answers: 10525 . 185602 , 102507 , 10089 and 20154 .
And Allah knows best.