If I were you I'd add up how much your food costs at the moment, include EVERYTHING such as buying juice bottles/coke/other drinks when out, the odd snack whilst out and about can cost £5+, then if you have a takeaway/eat out every week or every other week you're looking at an extra £20 at least, that;s £25 not including any food from a food shop!
I definitely am saving money as everyday used to buy something as a snack xx