A rental car is your cheapest option....the drive is pretty straightforward and easy. If you don't want to rent a car, then you will have to hire a car service. Because Sanford Intl. isn't as popular an airport (gets much less traffic) and is almost twice as far from WDW, the cost is almost double to get to WDW than it is from MCO. The cost is running about $180-200, without tip.
I love flying into Sanford...get some really terrific fares, but you do have to add in transportation costs once you get there.