Praise be to Allaah.
There is an important hadeeth from the Prophet (peace and blessings of Allaah be upon him) which explains this matter and says that this is one of the causes of doom. There follows the text of this hadeeth so that you may advise your wife thereby. Imaam Ibn Khuzaymah narrated in Kitaab al-Tawheed that the Prophet (peace and blessings of Allaah be upon him) delivered a khutbah (sermon) and spoke at length., in which he mentioned matters of this world and the Hereafter. He said that the first thing that led to the destruction of the Children of Israel was when the wife of a poor man would make demands for clothes and jewellery just as the wife of a rich man would demand.
Al-Silsilah al-Saheehah by al-Albaani, no. 591.
He said, this is a saheeh isnaad according to the conditions of Muslim.
You must remind her of the importance of being content and of asceticism in this world. Promise her that that you will give her more when Allaah gives you more, and remind her of what Allaah says (interpretation of the meaning):
“Let the rich man spend according to his means; and the man whose resources are restricted, let him spend according to what Allaah has given him. Allaah puts no burden on any person beyond what He has given him. Allaah will grant after hardship, ease” [al-Talaaq 65:7]