DL: DEN-MIA $210ai 4.6/5.3cpm
I know this fare has been around for a little while, but was playing around with it lately.
DEN-DTW-MIA-DTW-DEN for 4535mi at $210 or 4.6cpm
Unfortunately you'll have to overnight, only same-day I can find is:
DEN-MSP-MIA-ATL-DEN for 3974mi at $210 or 5.3cpm, which isn't too bad