There's not a lot of math to club membership, so don't strain yourself:
That includes one free book ($10). It also entitles you to 15% off all purchases. In addition, you also get quantity discounts (an extra 10% off orders over $100, 15% off orders over $200).
The bottom line is that my club membership has paid for itself in less than 3 months, and now the rest is gravy.