Clovis CC in New Mexico raised its rates to $89 per credit plus ~$100 in fees per course, which is still a good deal as they offer a lot of online classes. The University of Nevada-Reno is now $225 per undergraduate credit and is mostly on campus. Granted, if one has 5-6 weeks free in the summer, it would be less than $2500 for a domestic “study abroad” with 6 credits, books, room, board, airfare, and fees.
A number of universities don’t charge OOS fees for online students, though course offerings are often limited. Some reduce their OOS fees for summer classes, but check on the total cost as many have high per-credit tuition rates and fees to begin with. I chose to attend summer school in Reno because there were no OOS fees, per credit tuition was extremely low, and the two courses I needed were offered in the same 5 week term. Reno is also my favorite “little city” as it has great summer weather (sunny, 75-95 degrees, and no humidity) and an extremely low cost of living. Housing in a dorm room was $23 per night and a prime rib dinner was $5.99.
For most prerequisites and some standardized upper division courses, one can take the course almost anywhere. Find somewhere which offers the course you want at a reasonable cost.
My understanding of the University Scholars program was that one needed to have senior standing and have completed almost all of the required courses at the 100-300 levels. The Math and Science departments have been known to allow talented undergraduates to take graduate courses on a case-by-case basis without enrolling in the University Scholars program right away.