If you really need them, then you will pay almost anything for them. I don't recomend buying them at WDW. It is around 7$ for 20 diapers. Bring enough for the trip or go to the store. Resorts sell them too, for around the same price.
The baby care stations are great. They do have complamentry packages of ceral with formula. Baby food is around 10 cents more a jar. They have a good vareity (Gerber). Most other things you might need are also there. Disney is convenent, but you pay for it.
I thought I would cut down on amount of toys and pick some up there. They didn't have many toys for babies (our daugther was almost 9 months when we went.) Most that they did I already had and got for 1/3 the price at TJ Max.
I love the baby care stations. We found it was a great break to go and let her crawl around on the floor there and play with the toys there each day. It really helped her get a break from the crowds and me too!