When we took our first trip to fort lauderdale, my little guy was just turning 1. He was a hungry little guy and my milk was drying up so I needed to supplement with formula. I hated the consistency of the powdered stuff, so I used to used the liquid formula in cans. I had to pack those cans in my checked baggage. I paid $50 in weight surcharges for that trip. Fortunately, he consumed all that extra weight on the trip and we didn't need to bring it home again!
Fortunately, diapers are light and fit in a suitcase. I couldn't imagine the premium you would pay to buy those on board.