The minimum amount of mahr has been explained in the report in al-Saheeh (no. 1425) narrated from Sahl ibn Sa’d al-Saa’idi, who said: “A woman came to the Messenger of Allaah (peace and blessings of Allaah be upon him) and said, ‘O Messenger of Allaah, I have come to give myself (in marriage) to you.’ The Messenger of Allaah (peace and blessings of Allaah be upon him) looked at her and looked her up and down, then the Messenger of Allaah (peace and blessings of Allaah 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 Sahaabah said, ‘O Messenger of Allaah, if you are not interested in her, then marry her to me.’ He said, ‘Do you have anything?’ He said, ‘No, by Allaah, O Messenger of Allaah.’ 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 Allaah, I could not find anything.’ The Messenger of Allaah (peace and blessings of Allaah 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 Allaah, O Messenger of Allaah, not even an iron ring. But (I have) this izaar (garment) of mine, she can have half of it.’ The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘What can she do with your izaar? 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 Allaah (peace and blessings of Allaah be upon him) saw him leaving and called him. When he came, he said, ‘What do you know of the Qur’aan?’ 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 Qur’aan.’” 

This hadeeth 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-Shaafa’i and is also the view of the majority of scholars among the earlier and later generations. This was also the view of Rabee’ah, Abu’l-Zinaad, Ibn Abi Dhi’b, Yahyaa ibn Sa’eed, al-Layth ibn Sa’d, al-Thawri, al-Oozaa’i, Muslim ibn Khaalid, Ibn Abi Laylaa, Dawood, the fuqahaa’ of the scholars of hadeeth and Ibn Wahb among the companions of Maalik. It was also the view of all the scholars of the Hijaaz, 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. 

With regard to the question about the mahr of the Mothers of the Believers: 

Muslim narrated in his Saheeh (no. 1426) that Abu Salamah ibn ‘Abd al-Rahmaan said: “I asked ‘Aa’ishah the wife of the Prophet (peace and blessings of Allaah be upon him) what the mahr given by the Prophet (peace and blessings of Allaah be upon him) was. She said: ‘The mahr that he gave to his wives was twelve ooqiyah and a nashsh.’ He said, Do you know what a nashsh is? He said, she said it is half of an ooqiyah. That was five hundred dirhams. This was the mahr given by the Prophet (peace and blessings of Allaah be upon him) to his wives.” 

The scholar Ibn Khaldoon said: 

“The consensus of the scholars from the beginning of Islam and the time of the Sahaabah and the Taabi’een has been that the shar’i dirham is that of which ten coins weigh seven mithqaals of gold. The ooqiyah 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 (ijmaa’).”

(Muqaddimah Ibn Khaldoon, 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 Allaah 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 $396.7 

And Allaah knows best.