What is the appropriate mahr for prospective bride?

I am trying to learn what mahr is customary or recommended according to Islamic faith, customs, or tradition. Our prophet's cousin and first male Muslim revert, Ali ibn Abu Talib (ruh), was said to have provided a mahr to our prophet's daughter Fatima (ruh) in the amount of 400 Dirhams. I learned this information from our imam but could not verify the source. Since we should follow the example of our prophet (pbuh), Is this information correct, and what would 400 Dirhams from back then be worth today in US $? Please provide sources. Thank you.