Out of Denver, and I use the date grid on google flights. Frontier and spirit fly to Miami pretty consistently for a $80 round trip, but most flights are to like bentonville, AR or similarly random places.
The key is to people able to take a flight at any time and day, so you can always take the cheapest options. But also if you book 2 months in advance you can go anywhere in the Caribbean/central America on spirit for $100 or so.
