How do you decide which features, pages, components, modules to build and where to put the most effort and money?

You can’t build every feature and you shouldn’t put an equal amount of effort/time/money into every feature. I try to focus resources on whatever makes the most difference to users.

Here are a couple rules of thumb I apply:

If 20 people use feature X, it’s usually more important than if one person uses feature Y.

If feature X is used many more times a day than feature Y, put more time into getting it right.

If a web page is absolutely critical to a particular task, make sure it’s super-solid and well designed.

In one previous project I focused on the user interface for a data capture clerk over the boss’ favourite feature, because the data capturer used the app all day and his task was critical to the overall project success. The boss got over it.