Cheapest? LINX at around 5 bucks. But it would most likely literally take you all day to do so. Driving from Sanford to Disney would take at least an hour in good traffic. I'm sure you'd have many stops along the way using a buss system. Plus you'd have to be waiting outside in the elements in between. Not fun. However you can go to their website and plug in where you want to start and end your "trip" and they will give you the time it should take as well as the fair amount.
Taxi? According to taxifarefinder.com it would cost you $137.75, including tip.
My guess is that renting a car would be your best bet.