How much money should I expect to spend on a week vacation in Vegas?

I will renting a house with 10 other people to celebrate my girlfriend's 21st birthday. The house, rental car, and some basic groceries will already be covered. How much should I expect to spend for 7 days for the two of us? We want to see a Cirque de Soleil show and probably just hit up some night life and restaurants for the rest of the time. We might gamble one night, but not much.