It's pretty simple actually. We take into account the retail price of the product, the price we can sell it for at the end of its run with us, and the current age of the copy you have in your possession. If the copy you have is on its very first rental, the price is going to be pretty close to the retail price, because, hey, it's brand new! Likewise, if the copy you have is near the end of its rental life, it is going to be pretty close to the pricing you'd see in our Keeper section.