In order to answer the OP (for any other readers since OP himself/herself hasn’t come back): OP doesn’t have a choice. It’s not a matter of “ASU Barrett is cheaper”. OP knew that when entering the ED contract. Now, OP has to honor his/her commitment. The ONLY reason you can back out of an ED offer is if the school isn’t affordable. Duke is affordable (and having a stafford loan packaged in does not count as “unaffordable”). Therefore OP has to attend Duke.
Simple.
OP doesn’t seem to worry that he’ll be blackballed (note that if OP backed out of Duke, Duke is forever off his list since they ask “have you ever applied to Duke”…), nor seems to have dwelled on consequences for his high school - subsequent classes, current classmates, and guidance counselor.