Lulu is having a really good sale at the moment, with 25% off. If you've been thinking of buying a printed copy of the Core Rules 4th printing, now is a good time. Most of lulu's discount sales are more in the neighborhood of 10-15% off.
The coupon code is CYBERMAY305
EDIT: Dropped prices by an additional $1 - lower price shown below
Softcover: $14 with my discount ($15 normally) Price would effectively be $10.50 with lulu discount
Hardcover: $24 with my discount ($25 normally) Price would effectively be $18 with lulu discount
If you're buying 5 copies of the same book at lulu, they give an additional discount of 5%. So if you're buying for a gaming group it's better to have one person order the copies for everyone instead of having everyone buy separately.
Also note that this doesn't reduce shipping costs, only the cost of a printed product.
You can only use the coupon code ONCE, so your first order should contain everything you want.