Ah but do they get more on a 5 week month?
seriously, it sounds good. My only concern is that if they weren't as sensible as I'm sure they are you're sort of pressurising them into spending less on Lunch to buy nice things elsewhere, I could see that going wrong in some cases. Ring fenced cash for lunch where if you don't spend it you lose it might be a better plan. After all there are a good few youngsters who are already getting pressurised into skipping lunch to look good, offering them a cash incentive to do so doesn't sound quite right to me.