There is no limit to how much you can earn at CafePress.com. We have a base price for our products and the markup you set above the base price is your royalty. For more information about our base prices, click on the link below: http://www.cafepress.com/cp/info/help/base_prices.aspx
When customers buy directly from your shop you will earn the markup amount you set. But keep in mind we set the retail prices in the Marketplace. When customers buy from the Marketplace designers will receive a 10% royalty of the final retail price.