I think it's a nice idea to pay your kids for things they do. Not everything can be money. It can be stuff like dinner, lunch, a trip to Six Flags, etc.
If your kid has a Wii, you can offer to buy them a game of their choice (as long as it meets your rating requirements) if they get all A's.
If you're good enough at math yourself, you can easily figure out how much you can afford to pay them for the whole year.
Since most schools have six 6-week periods, that's 6 times the money you will likely be spending.
So, if you're willing to spend $300 for the whole year... that's $50 per 6-week period. If they don't get those A's, that's more money you've saved... and can be used towards a different kind of agreement such as helping around the house, etc.