The unnamed prince had approached the Hurricane Sandy relief organiser, saying that he would make donation if he could talk to the "Twilight" actress.
"How much?", Stewart asked Weinstein after which the producer negotiated the number with the prince and the amount was raised to $500,000.
The money was paid in advance in form of cash.
In addition to the Sandy storm relief, the actress contributed to donation for people who were affected by devastating earthquake in Haiti in 2010.
The 23-year-old auctioned off herself by offering a chance to hang out with her at the Sundance Film Festival.
Stewart also auctioned off several items and donated the money to fight child prostitution following a research about teenage prostitute for her movie "Welcome to the Rileys".
Weinstein was at the Toronto International Film Festival to screen the Hurricane Sandy benefit concert film "12-12-12".