I would use the Bitcoin Mining Profitability Calculator
http://www.bitcoinx.com/profit/Based off of your initial cost $4000 (assuming that you don't have to buy extra equipment), an average of 685MH/s a card (if you OC a little), the current difficulty. You're looking at 0.153 BTC/24h, and running 1000W (max), it would take you a year to generate that back.