Regarding the savings at a university, assuming that the university took AP credits, the way you would save is if you had a bunch of credits (at least 16) and would get your degree in less than 4 years. Most schools charge by the semester or quarter for full-time attendance and not by the unit, so the savings per class is more or less invalidated. The other thing to consider is that many STEM-related majors have course and sequence requirements, not to mention prerequisites and competition for registration that will be very hard to squeeze in in less than 4 years. As such, many people with AP eligible credits will wind up not using them in college.
I think people overthink it when it comes to APUSH. As long as the teacher is good, there’s no reason not to take it. It’s a medium difficulty course, not too easy but not too hard.