During the year, the best deals I've seen are 15% to 20% off. I only pull the trigger when I can get 20% off (40 for 50 or 80 for 100). Last year's BF deal was mostly 15% off so not enough to entice me to buy at that time.
PayPal and Groupon regularly run the 20% off promos on digital iTunes purchases. In store I've seen Target and Walmart and on the rare occasion do 20% off at Best Buy/OfficeMax/Depot/Staples, but mostly its just 15% off.