How Much Money Should I Give My Wife Each Month?
Question

Is the wife entitled to a monthly allowance that is just for her, especially if she has no source of income except what her husband gives her?
Praise be to Allah.Obligation to spend on wife 
What the husband must do is spend on his wife on a reasonable basis, which includes the costs of accommodation, food, drink and clothing, according to scholarly consensus, and includes medical treatment and medicine according to the more correct opinion.

With regard to accommodation, that is because Allah, may He be Exalted, says (interpretation of the meaning):

“Lodge them (the divorced women) where you dwell, according to your means, and do not treat them in such a harmful way that they are obliged to leave.” [At-Talaq 65:6]
If this refers to one who is divorced, then it is more applicable to one who is not divorced.

With regard to food, drink and clothing, that is because Allah, may He be Exalted, says (interpretation of the meaning):

“but the father of the child shall bear the cost of the mother’s food and clothing on a reasonable basis.” [Al-Baqarah 2:233]
And the Prophet (blessings and peace of Allah be upon him) said: “Their rights over you are that you should provide for them and clothe them on a reasonable basis.” (Narrated by Muslim, 1218)
It was narrated that Mu`awiyah Al-Qushayri said: I said: O Messenger of Allah, what are the rights of the wife of one of us over him? He said: “That you should feed her as you feed yourself, clothe her as you clothe yourself, not strike her on the face or curse her, and not forsake her except in the house (as a form of discipline).” (Narrated by Abu Dawud (2142) and Ibn Majah (1850). Classed as authentic by Al-Albani in Sahih Abu Dawud)
Ibn Rushd (may Allah have mercy on him) said: 
“The scholars agreed that the rights that the wife has over the husband include maintenance and clothing, because Allah, may He be Exalted, says (interpretation of the meaning):
“but the father of the child shall bear the cost of the mother’s food and clothing on a reasonable basis.” [Al-Baqarah 2:233], and because the Prophet (blessings and peace of Allah be upon him) said: “Their rights over you are that you should provide for them and clothe them on a reasonable basis”; and he said to Hind: “Take what is sufficient for you and your child, on a reasonable basis.” 
With regard to maintenance, the scholars agreed that it is obligatory.” (Bidayat Al-Mujtahid wa Nihayat Al-Muqtasid, 2/44)

With regard to medical treatment and medicine, please see the answer to question no. 83815 .

How much money should I give my wife each month?
If the husband does that, he has done what is required of him, and he is not obliged to do anything beyond that, or to give his wife a personal allowance , unless he chooses to do so.
Virtue of being generous towards one’s family
But the husband should understand the virtue of being generous towards his dependents , and treating his family kindly, and should understand that whatever he spends on his wife , he will be rewarded for it, for giving gifts increases love. So if he is well off, he should not refuse to give her this allowance, especially if she does not have any other source of income.
Al-Bukhari (55) and Muslim (1002) narrated from Abu Mas`ud Al-Badri (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “If a man spends on his family, seeking reward for that with Allah, then it will be counted as an act of charity on his part.”
And Muslim (994) and others narrated from Thawban, the freed slave of the Messenger of Allah (blessings and peace of Allah be upon him), [that he said]: “The best dinar that a man spends is a dinar that he spends on his dependents, and a dinar that a man spends on his mount (for Jihad) for the sake of Allah, and a dinar that he spends on his companions (in Jihad) for the sake of Allah.”
Abu Qilabah said: He began by mentioning his dependents. Then Abu Qilabah said: What man earns a greater reward than a man who spends on his young dependents, whom Allah keeps from needing people or benefits or makes independent of means through him?
Al-Bukhari (1295) and Muslim (1628) narrated from Sa`d ibn Abu Waqqas (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said to him: “You will never spend anything, seeking thereby the pleasure of Allah, but you will be rewarded for it, even a morsel that you put in your wife’s mouth.”
And Allah knows best.