Yes, but even with the "more reasonable" prices at Backstage food locations, you'll still be better off financially if you bring your own lunch.
For example, I can go to the MK Cast Cafeteria ("Mouseketeria"), and buy a Subway Sandwich. 6" turkey sub sets me back about $4.50. I can go to an off-property Subway, and get a footlong sub for $5. I can go to the grocery store, buy a loaf of bread, some sliced cheese, and some deli turkey for $5, and that feeds me for 3 or 4 days.
And, as Glendalais says, you're technically not supposed to buy food on-stage during your shift. Does that mean it never happens? No. There are some costumes that have pants/shorts that look sufficiently similar to "civilian clothes" that if you bring a spare T-shirt, you'd probably be able to get away with it, but be careful ... if a manager catches you, you could get in trouble.