It really depends on the school. I would say that it is often cheaper if you do it the right way. In my experience at the University of Massachusetts Amherst, i often took classes during the regular fall and spring semesters through the school, but then would take a few classes at local community colleges online during the summer in order to avoid doing a fifth year of school. I found that by taking the classes at a community college, it was much cheaper (and easier) than if i had taken the classes at Umass. It also counted for the same amount of credits and helped me finish faster. It also would allow you to get the college experience, and save some money since your paying tuition yourself.
I would recommend applying in state and then asking your assigned advisor about summer opportunities to earn credits outside the university. Most large state schools allow it.