Yr 10 my parents paid for half of my PC (wasn't incentive, they told me after I got the grades). Yr 11 I got a new bike. AS levels I got a nice dinner out.
At GCSE people aren't taking it as seriously so need an incentive which they really do like the sound of. I remember with yr 10 GCSE's we were recommended 5 hours a day decision during the half term holiday before them. Everyone including me moaned about that, do nearly that currently before the exam period and I don't mind it.