Calgary to Miami - $381 roundtrip after tax - throughout winter
There seems to be one of those freak price drops on United at the moment. I do see these on rare occasion, where they affect prices throughout many months in their schedule, all the way up to a year ahead.
They've dropped their prices to Miami to be nearly 50% cheaper than the next cheapest airline.
For comparison, flights to Orlando and Fort Lauderdale are usually priced about the same as Miami, and they're currently at $746 and $811 roundtrip respectively for the cheapest flights in March.
Here's a sample search in March to Miami...
http://www.kayak.com/flights/#YYC-MI...03-28-flexible
screenshot:
Cheap ones seem to be available in nearly every month at the moment. Try Tue/Wed/Sat to Tue/Wed flights for the best chance.
If you do a search on Kayak, and continue on to a booking site, you may want to choose Expedia or Cheaptickets (owned by Orbitz). The link from Kayak to United.com's site may give you headaches since it will only accept US credit cards. (Although you could just use Kayak to search for the dates and then perform a search on United.ca - which does accept CDN credit cards).
Also note that some of the United flight combos have more than 1 stopover, so deselect '2+ stops' on the top left of Kayak after you perform a search.