Hi Tina, and welcome back to planDisney!
I am unsure which transportation options are the least expensive because so many factors are involved, like dates, number of guests, etc. However, I know that some options can fit into any budget. Budget is important especially when you vacation at
Walt Disney World Resort.
The first option would be to utilize the services of MEARS Connect. The round trip cost is pretty reasonable for the service that you get. Another similar bus service, The Sunshine Flyer, operates out of the Orlando International Airport for the resorts at Walt Disney World. I recommend checking them out and comparing the costs to accommodate you and your family. Another quicker option that might fit into your budget would be to use one of the smartphone ride-share apps, like Lyft. You can always rent a car, but that would involve additional parking fees and gas.
Come back and let us know how we can assist you further before your arrival.
Have a magical trip!
Raquel