I have 2 at school full time and a third 3 days a week.

Bread/wraps - $12 per week
Fruit - $20-25 per week
Home made muesli bars - $5 per week
Fillings for wraps/sandwiches (ham, vegemite, lettuce, cheese etc) - $8
Home made pikelets/muffins - $3 per week
Cheese sticks - $7 per week
Yoghurts - $6 per week

Total - $61 per week! Wow never worked it out like that before. Upon saying that though some of those costs can be averaged over a couple of weeks - eg the yoghurts and cheese sticks are not daily and my muesli bars usually last 2 weeks too. I suppose also some weeks it may be more as I may buy a packet of muesli bars if I haven't had time to make any.

ETA - Our school only has canteen one day a week and DD1 orders sushi at a cost of $4.50 and DS a toasted sandwich $3.50 so thats an additional $8.