In some cases, it’s easily quantifiable.
For example, the Macbook can optionally come with exactly $100 of Black included.
It is felt by many in the Apple camp that software that requires a second mouse button is poorly designed. And it’s often true. Nevertheless, lots of people want to make poorly designed software, and most mice have multiple buttons by now, so it is supported.
I tend to agree with Apple’s historical stance for a couple of reasons. First, the decision of having two buttons is fairly arbitrary. The design choice of one button is equally valid as that of 2, or 3, or more. But you never hear people complaining about why Windows doesn’t support 8-button mice. In fact, most standard mice have three buttons, now, one of which is really three buttons (scroll wheel up, scroll wheel down, middle click). Yet, I can do all the things I need to on my Mac with just the trackpad. Two fingers is context click. Two finger motion is scrolling (and I get horizontal scrolling for free), and Cmd-click is “right” click, or “context” click. Why add another button to the mouse when you’ve already got a keyboard full of buttons that aren’t being used while you’re using the mouse? From a design standpoint, the philosophy of “We need to add some more functionality, let’s add another button” is inelegant, harder to learn, and becomes unwieldy quickly.
Also, if you’ve ever had to do low-level support or teach total newbies to use computers, you’ll understand how confusing two mouse buttons are to someone with no context.