Completely agree. Another dimension along which it varies is the genre of the book. A new romance novel does not need as much research or effort put in as many of the academic books (not textbooks). Such books need extra support for the authors and need more effort in editing and peer review. However they are also pirated at the same price - 0.
Also with digital goods, once you purchase them, you get a life long license to download them from the vendor. With the physical goods, it is a one time sale. So in effect with every digital sale, vendors have a much longer liability to provide you the access to content. Maintaining such repositories of content has ongoing costs which must be worked into the price of digital goods.
> Also with digital goods, once you purchase them, you get a life long license to download them from the vendor.
Who says so? I know it's true for O'Reilly, but most music providers, for example, don't work this way. If a vendor doesn't want to take this responsibility, then it can just simply state what access it is willing to provide.
Also with digital goods, once you purchase them, you get a life long license to download them from the vendor. With the physical goods, it is a one time sale. So in effect with every digital sale, vendors have a much longer liability to provide you the access to content. Maintaining such repositories of content has ongoing costs which must be worked into the price of digital goods.