Don't make halal expensive and haraam cheap.
To begin with, the Mahr (dowry) is the right of the woman who is getting married, not her parents'. It should be given to her; "And give to the women (whom you marry) their Mahr with a good heart" (An-Nisa 4:4).
The Prophet salaAllahu alayhi wasalaam said: "The best of mahrs is the simplest (or most affordable)" - Al-Haakim and Bayhaqi; Saheeh al-Jaami' 3279. And, "The best of marriage is that which is made easiest" (Ibn Hibbaan; Saheeh Al Jamii' 3300).
There is no certain limit for Mahr but the guidance of the Prophet salaAllahu alayhi wasalaam i.e the Sunnah, is to make it simple and not too much to the extent that the marriage becomes a burden.
Ali radhiallahu'anhu said: I married Faatimah (radhiallahu'anha) and said 'O Messenger of Allah, let me go ahead with the marriage' He replied: "Give her something" I said: I do not have anything. He said: "Where is your Hutami shield?" I said: I have it with me. He said, "Give it to her" (And that was her Mahr, the leader of the women in Jannah; Saheeh An-Nasaa'i 3160).
Once a woman came to the Prophet salaAllahu alayhi wasalaam and said: I dedicate myself to you (for marriage). The Prophet remained quiet. She sat down, then a man said, "O Messenger of Allah! If you do not wish to marry her, marry me to her" The Prophet said, "What do you have to give as dowry?" The man said I have nothing but Izaar (waist sheet). The Prophet said, "If you give her your Izaar, you will have no Izaar to wear, so find something" The Prophet encouraged him "Find (something) even if a ring of iron" But the man came back with nothing. The Prophet then said, "Have you memorised something of the Qur'an?" The man said, "Yes, I memorise such and such" The Prophet concluded: "I give you to eachother in marriage for what you memorise of the Quran" (Bukhari & Muslim).
A'isha said, "The mahr that he (the Prophet) gave to his wives was twelve uqiyah and a nashsh (A nahsh is half of an uqiyah). That was five hundred dirhams then (Muslim 1426). This is approximately 1487g of silver or 395 dollars - affordable to those