Really, it depends upon the item. Some items are pretty much standard "convenience" store price + maybe 10%, some items are "Disney" priced. Any alcoholic beverage is Disney priced, and then some. Snack items, like doughnuts and pastries, are priced like a "normal" upscale grocery store. Eggs, lunch meats, bacon, etc. seem to be a little higher than the average convenience store. And of course, any Disney branded snack is quite high.
Soda and water have a high mark-up.