So are you just wanting to include the new ProductCost in the cart to record, or do you need to add it to the cart then alter the calculation that is used for the total price?
If you still need to alter your cart object then you should be able to follow the instructions in the pdf you referenced for that part. You would then need to alter the add to cart server behavior to make use of this new value from the db. Make sure that you backup your existing WA_eCart folder before trying to update the cart. Please post back and include more specific details about what you would like to do with the price and cost values once you have them both available in the cart.