FYI, next week is the last week of classes. However, it is really a nice time to visit. Pollen and allergies has been bad recently but hopefully the rain will clear things out.
Chapel Hill does have four seasons but the winters tend to be very mild.
As far as the cost difference, CH does try to keep the costs down. The meal plan are flexible from no meal plan to full meal plan. They have an excellent and free bus system so living off campus after Freshman year is very doable. I believe dorms are now open during breaks (except XMAS) so it is possible to stay on campus during breaks. You will need to verify this.