@El_Heffe said:
Since you can't buy it (sold out) the price you pay would be zero, which is a 100% discount, as shown on the website.
You can't buy one of these, either: http://store.valvesoftware.com/product.php?i=STKT01 -- see what happens when you add it to cart. Note that "rate" (a.k.a. price per item) is $0.00, yet the subtotal is fine..