Renting a car is probably the most economical, but it's also a bit of a hassle. If you rent from the WDW car care center (Alamo) they will pick you up if you are staying at an an onsite resort. You have to call ahead and schedule a pickup time.
We prefer to use a private car service to get to Port Canaveral. Last year we had a group of 8 in a 10 passenger van and it ran about $20 a person including tip.