I have a question that hasn't been asked:
Are you doing this so that you have a couple of extra days in the magic, or just trying to shave a couple of days (and expenses) off your vacation?
If it's the first, you won't be saving any money since you will be spending it in Florida.
If it's the later, it may be worth the cost of booking flights (for at least part of the way)
We live in Winnipeg Manitoba and will quite frequently drive down to Grand Forks or Fargo ND or even Minneapolis to save on airline costs (driving 2,3 or 6 hours is way more doable than 30+ hours from home).
***Follow my logic here: if you have 2 weeks vacation but you are spending 6 days driving (plus another day at each end of the drive recovering from that drive), this gives you 7 days (maybe 8) to enjoy your vacation before you have to buckle down and make the drive back (We always hate the drive home - are we there yet? are we there yet?! ARE WE THERE YET!?!)
***Personally, we vacation pretty hard and need a vacation from the vacation when we get home, I don't know if I could drive for 3 days after 7 days in the parks.
***Now if you were to just plan on 7 days in the Magic, look for a discount flight from an airport near home or one that is just across the border into the USA and only plan out the 7 days plus 1/2 day air travel at each end, you may find that you save your sanity/marriage/kids/etc at a cost that isn't a whole lot more than the 14 day driving vacation.
JMHO