Where do the original prices on the products come from?

Next to a product listing, you'll see its price and how much you are saving from the original price. 
The original price displayed on a product listing is the price of that device when sold new. If a device is no longer being sold new, the comparison price is how much the product sold for when Apple first released it. Of course, this is only an indication since the selling price of a new device often drops after the release of new generations.