You can see all of their "base pricing" on this page:
http://www.cafepress.com/cp/info/sell/products/viewall
You set your markup above their base price and your commission is the difference between the two. The white t-shirt is $13.99 and the have a lighter weight t-shirt with a base price of just $8.99 printed.
Some peopel sell t-shirts for $15.00 each, some sell t-shirts for $20+ each and make a bigger profit.
You also have to consider, that the base price is not just the cost of the garment and the printing. They also setup your online store, process credit cards, host your store, handle customer service, have a heavily promoted marketplace.
So basically, they take all of the risk out of setting up an online t-shirt shop (little to no upfront investment), and the tradeoff is, they pay for their service with the base prices (you don't pay if you don't sell...at least with a basic store).