American and Delta have low one-way fares from New York JFK to Miami and reverse at $54 on AA and $69 DL on Tuesdays and Wednesdays from November to February. While Delta is more expensive at $68, there are far more low fare dates at $68 with DL than $54 AA fares from LGA.
Both airlines offer more low fare dates at $69 from JFK.
United is $62 to $88 each way for Newark EWR – Miami MIA.
American Airlines New York LGA – Miami MIA nonstop
Delta Airlines New York JFK – Miami MIA nonstop