thanks ladies for all your replies. I have now put a limit of two pieces of a fruit each a day.

DS14 was not impressed but I told him it was 2 or nothing!

I dont mind them snacking on the fruit as it is a much better option for them, but when its gonna cost me $20 a day/$140 a week just on fruit I think we have to reassess things.
Especially the older boys as they can make themselves something to eat. There is usually some kind of leftovers in the fridge, there is always cereal, noodles, spaghetti, plenty of bread (I buy 12 loaves a week), crackers etc but they are just been lazy and grabbing something that requires as little output as possible

I dont feel so bad now for putting restrictions on it when I know there is still plenty for them to have if they are hungry.