So the most affordable way (and reliable) to get to southern destinations is Southwest and JetBlue. Today, JetBlue opened its availability calendar 'til February 14th. Back on May 18th, Southwest opened theirs until January 7th.
To begin, I LOOOOVE SWA. They are a well established airline with a very good reputation, even lately. The flexibility of cancelling your ticket even an hour before the flight, and getting a SWA credit towards your next flight is hands-down better than their competition, even their US competitors, so much more so compared to CDN competitors.
Love that no matter what class you fly or how much your ticket cost, you get 2 checked-in bags per person for free. You also get snacks and drinks for free. That's already better than pretty much the best of all airlines that fly within North America.
I haven't bothered reporting the availability calendar opening as far as January 7th, as I have been back and forth with Customer Relations of SWA. I've been finding that the website claims that flights in the cheapest class "Wanna Get Away," (WGA) have been listed for many flights as "unavailable" or "sold out". Misinformation. In fact, when a flight to San Juan from Buffalo is going for $704 one-way, does anyone in their right mind believe that these flights, at the lowest economy fare are "sold out"? Keep in mind, I knew what day and approximately what time the flights went on sale, and I KNOW there is no way this "sold out" or unavailable status happened in about 3 minutes of the tickets going on sale, so it turns out, SWA has a new booking system (started around May 9th) and SWA's marketing department is being quite crafty as to when they're actually going to release the bulk of their WGA flights, or lower rates on the WGA flights that they have already released. No one is paying $414USD for a one-way ticket from Buffalo to San Juan in WGA economy when they can get a flight cheaper out of Toronto in CDN$. First I was told that the tickets would be released in the next one to two weeks---well, that's past now and they still haven't released them. They will at some point, or they'll find they're flying empty planes. I don't think this is a glitch in their knew booking system. I think that they hope there are people foolish enough to pay these rates, and then they'll drop them. Crazy thing is if anyone knows the rules with SWA, they know that if the price drops with a sale (or for any reason), SWA will give you the difference as a sort of gift card that you can use for future SWA flights--and if you booked it with points, they'll refund you the differential, so they have every reason to release the cheapest seats soon so they can fill their planes faster.
JetBlue I am not as knowledgeable about but I suspect they are doing the same thing in the sense that they are not releasing all their flight availability nor their cheapest rates, especially during the holiday season. Wait it out and check in regularly to see if the flight you're looking for becomes available, and if the time you want to leave is available, you may wish to book it now and hope the price drops before your flight leaves.
At least, with Southwest, they apparently drop their ticket rates, sometimes even hours before the flight.