For the lazy or the math disinclined - I'm both!
Link: online DVD rentals--price per rental calculator.
Instructions
1. Enter the number of rentals your received per billing period.
2. Not necessary, you may enter any sales tax charged for service
3. Select your plan--Includes Blockbuster, Netflix, and GreenCine
4. Click the "Calculate" button
5. Your price per rental will appear in the last box