If the marriage was performed without the information and permission of the bride’s family, then that marriage is invalid. Please see the following fatwa about the mehr in case of invalid marriage:
If the marriage is performed under the supervision of the bride’s family, then there are two cases about the amount of mehr to be paid. Relevant verses are as follows:
“(Oh men!) There is no sin upon you to pronounce the word of divorce to women before settling on the legal share of bride and touching them (consummation). Give benefits to them. Those who have ample means must give according to his condition and those who have restricted means must also give according to his condition, in compliance with the known terms (of the Quran). This is a duty for those who behave kindly.
(Oh men!) If you divorce the women whose legal shares of brides you have settled before touching them, then you must pay half of the settled amount, unless the women or those who hold the knot of marriage in hand pardon their rights. (Oh men!) It is more appropriate for you to pardon it (your right) in order to protect yourselves (for taqwa). Do not forget the superiorities over one another. God sees whatever you do.” (Al-Baqarah/ The Cow 2:236-237)
If the divorce happens before settling the mehr, then the groom should give bride a gift which she can benefit from.
If the mehr was settled, at least half of the settled amount must be paid to the bride. Yet, the Almighty God recommends men to give all of the settled amount rather than half of it. This is closer to piety for men.