Do any of your kids have to do chores and/or get an allowance yet?
Miles isn't a horrible helper, but he's not great either. He will help but there isn't anything he does consistently (or does without me asking). Anyway, my friend does something like this and I was going to do it too... Each of her kids picked out a toy worth about $15 and they are working towards it. She made (kids made actually) a chart with squares and each square is worth 50 cents. When they do a task, they color it in and when the charts are full, they get their toy.
I was going to do something similar. I like it because one I won't have to take him to the store with a bunch of quarters and two, he doesn't really understand the value of money quite yet (like a quarter is worth 25 cents means nothing to him, you know). I was just going to make him a chart though and when he gets about 20 squares give him money or take him to the store or whatever.
Anyway, I mentioned it to him this morning and it thought it was a really neat idea! I have a feeling he will be really helpful when it wants something but not very helpful when he is content, lol.