It is unIslamic for a Muslim woman to set a huge demand for herself, with the intention of deterring suitors of humble means. Islam does not require husbands and wives to come from the same social strata or income brackets - although this may often seem to be advisable. Islamic compatibility is based on religious faith and mutual respect, not on money, caste (another Hindu custom), class, background, nationality, etc.
It is just as unIslamic to demand a huge mahr, generally beyond the husband's means, based on the intention of checking the husband from ill-treating his wife, or wrongfully or causelessly divorcing the wife, or preventing him from remarrying another later - the reasoning being that in cases of divorce the woman can demand the full payment of the mahr. The fixing of a substantial mahr for the above purposes rests on the supposition that the mahr has to be fixed at the time of marriage, but not handed over until divorce - which gives it a supposed 'deterrent' value. This is unlawful in Islam, for in this case the wife has no use or ownership of the mahr during the time of the marriage.
If the prospective husband is not a wealthy man, a generous wife may choose to accept very small mahr, but this has to be her own free choice. She should not be coerced or have pressure put on her in any way. Some of the Prophet's female companions accepted their husbands' conversions to Islam, or memorising of ayat of the Qur'an, or giving education to others as their mahr.
The mahr has to be fixed taking into account the bridegroom's position in life. That is, it should not normally be more than he is easily able to afford, whether it be a lump sum or some article of value. Jurists have different views on what the minimum amount should be, but all agree that it should be substantial enough for something to be bought against it. In other words, any amount which is sufficient for a purchase is acceptable as mahr.
The husband may be loaned money by his father or family, but it must be repaid. In the case of Nabi Musa (the Prophet Moses), when he left Egypt for Madyan he married Safura the daughter of the Prophet Shu'ayb. His mahr mu'ajjal was settled and paid off by binding himself to grazing his father-in-law's cattle for ten years without wages. Presumably Shu'ayb had paid Safura on Musa's behalf.
A good woman might agree on a low mahr if she wishes, or none at all, according to the circumstances of her husband. Once fixed it is fixed, and legally binding - so it is good practice to have it written down and witnessed on a document. The wife should take advice on her decision, and not be blinded by emotion, or coercion, or fear, or family pressure. If any person pressurises a woman into a decision she might not have otherwise made, that person will be held to account in the Life to Come, even if he 'got away with it' on this earth.
One recorded hadith suggests that 'the best woman is the one whose mahr is the easiest to pay.' (al-Haythami, Kitab an-Nikah 4:281).
However, it is sensible for a wife to accept a reasonable mahr, as this becomes her own property as stated, and is hers to keep should the marriage fail and end in divorce.