I found seats departing Atlanta ATL on Friday, February 8, returning from Miami MIA on Monday, February 11.
Similar fares between $122-$136 are available for flights departing on Fridays, and returning on Mondays, on a few weekends in early February and early March. You'll find even lower fares for mid-week travel, but we all know it's easier to get away for a quick weekend than it is to fly during the middle of the week.
There are also lower fares on Frontier if you're able to travel super light with just a small personal item. American's Basic Economy fare includes a normal carry-on bag, but you'll have to accept a random seat assignment at check-in for this price. A Main Cabin Economy fare for the mentioned dates is $199 roundtrip.
Need a room in Miami? Here's what's available for the mentioned weekend dates on Hotelwatchdog.