Cornell is one of the universities in the US that has the most diversity of PE courses that you can take, so it's a good opportunity to take them!
https://classes.cornell.edu/browse/roster/SP24/subject/PE
All students can enroll, but since there are limited spots, they get full really quickly. Hence, the university also prioritizes students differently. Undergrads can enroll in PE courses earlier each semester (Seniors to Freshmen, in that order) than Grads/Professional students. So, Grads/Professional students or Freshmen/Sophomores often get waitlisted; don't lose hope though, many have gotten in from just registering for enrollment as soon as it opens and being patient on the waitlist! Since a lot of courses require a fee, many students who are initially enrolled may drop the class if they decide to not pay.
Varies. Some of them are affordable but honestly, some of them are not cheap e.g. skiing, golf, or scuba diving. Overall, they are probably still cheaper than prices that you find if you look for a similar course outside of Cornell in the US, though.