My friends and I purchased a snack box on a flight from Orlando to Philadelphia in November. SafetyMom sure is right, they do run out. We were sitting at about the halfway mark of the plane, and we got the last one. It had a small sandwich, and I think it had three or four different snacks. I don't remember the price, it was about $5 or $6. We were each going to purchase one, but the three of us shared it. It was a nice snack. They also had a lunch box, which had a bigger sandwich and some snacks, was $10. They were gone too.
We were only on a 2 hour flight, so it didn't matter to us. I think if you are on a longer flight and want to make sure you have something, you would be much better off purchasing something at the airport.