I would say that Italy is probably cheaper than France, depending on where you are and what you do of course. It's cheap and easy to get a nice slice of pizza or a panino for lunch or dinner; so fast food is not automatically a bad option compared to buying ingredients and cooking/preparing a picnic. As everywhere, it's the little things that add up. Maybe take a cool bag and some freezer blocks so that you can buy cold drinks at the supermarket and take them out with you. Be aware if you're planning any beach trips that a lot of the beaches are taken up with bagni where you pay - around e15/20 per day - for a lounger/umbrella. Also, if you do go to a sit down restaurant there is a cover charge. I would say £300 for a week would be OK for one adult and a child unless you want to eat out a lot or do lots of expensive activities.