Imagine telling a 30 year old that they are going to have to spend three years doing something before they get a reward. How would they react to it?
For some of us, at age 22 we decided that it would be ok to spend 5 years of our lives doing something before we could get a reward: a PhD. 5 years was more that 20% of our current life, but we did it anyway.
As far as I'm concerned, paying kids for grades delivers the message... "If you work hard, you will be rewarded." School is the equivalent of work for kids.
Won't it teach them that doing well in school will get you rewards? If you want to teach them that hard work is rewarded, why not reward work, either chores or studying. Otherwise, won't your 120 IQ kid who gets straight-As without studying look at his 80 IQ C student brother who studies 40 hours/week learn that hard work is never rewarded, only intelligence?