This question gets asked a lot here in the transportation board
The best answer seems to be to rent a car.
http://www.disboards.com/threads/getting-to-disney-from-sanford-airport.3505641/
http://www.disboards.com/threads/what-is-the-best-shuttle-service-from-sanford.3500875/
http://www.disboards.com/threads/transportation-from-sanford.3499855/
http://www.disboards.com/threads/alamo-at-sanford-sfb.3482479/
http://www.disboards.com/threads/car-service-from-sanford.3466411/
Depending on which route you take from SFB to WDW its 45-60 miles and an hour or more on the road (plus a few dollars in tolls if you use 417)
Allegiant has a shuttle that you can book, I believe it's $72 round trip per person.
It can be an OK option is you are flying solo or maybe as a couple.
It can get really expensive if you have to shuttle the whole family (family of 4 would be about $300ish with tip).
Towncar is going to run you $220-280 depending how large of a vehicle you need for a round trip (usually up to 4 passengers in Towncar, 6 in an SUV).
Uber is a possibility but timing could be an issue (due to how remote SFB is) and pricing can vary (Uber XL from SFB to WDW shows $70-95 each way = $140-190 RT). Only problem I see with using Uber for this will be if there are no drivers in the area or drivers who want to pick up a long haul fare like this.
Taxi's are plentiful and easy to catch outside the terminal but cost wise it's probably your most expensive option (you are looking at $125-140 each way = $250-280 RT)
A rental car would be our choice and can be a very reasonable option both cost and convenience wise, depending on the vehicle you want to rent and the time of year you will be travelling.
There's a couple of options here:
1) rent a vehicle for the whole time of your visit (gives you the flexibility to explore outside the Disney bubble if you want)
A Weekly rental can be less than $100 for a mid-size/Full size vehicle in the off season, maybe $120-150/wk in high season (plus gas and tolls)
2) rent 2 one way rentals with a company that has a location on Disney property like Alamo (at CCC). One way rentals can usually be found for $25-60/day
One rental from SFB to your resort and then drop of the vehicle at the Care Care Center (Alamo rental). CCC will shuttle you back to your resort.
For your return flight, book a second one-way rental from CCC to SFB .
