I need to ask a few questions:  How often are you watering, are they sitting in a saucer with water?  Have the pots been on the Florida soil at all?  The flies look like fungus flies which is caused too much water.  When was the last time you fertilized?
If you get back to me tonight we can get this sorted out.  A

Hi Andy,
thanks for getting back to me. Yes, they are definitely fungus gnats as I had been watering twice a day to help survive the South Florida sun and the soil in the pot was always moist. What is the best and most cost-effective solution for getting rod of them?

The only way to get rid of fungal gnats is to stop watering so much.  You can use sticky strips but they will not go away unless the environment changes.  Consider carefully transferring your tomato plants into a new 5 gallon painter's bucket with drainage holes drilled 2 inches up from the bottom on the side of the bucket.  It allows the lower part of the soil is retain a small amount of water but more importantly it allows more soil for your deep rooted tomato plant.  These guys do not last in a shallow situation.  More soil means you do not have to water until 2 to 3 inches of the topsoil is dry thereby alleviating the necessity to over water. They are heavy feeders too!  Is this something you can do? A