When shopping for an alternator, you need to get two numbers from them, and some can provide an actual output curve.
* amp output at (or near) idle.
* RPM needed for max output. (What does it do low, and how high do you have to go)
If they can't or won't provide those then move on.
A good alternator is going to do max output around 2K RPM. A cheap alternator is not going to do full output until over 3K rpm. Well where do you drive at? What rpm are you at when you are cruising? Most people don't cruise over 3k. On a crappy alt, you won't rev high enough to get that output. That's the difference between a $200 alt, and a $600 alt.