Getting kids to practice

We have been having problems getting Robert to practice his piano on a regular basis. But money talks to him. We took the price of his lessons and divided it throughout the days between his lessons. We pay $6 for a lesson once every 2 weeks. We also planned 2 days off incase we get really busy or if he has a lot of homework. Then we offered him money for each time that he practices to pay for his lesson. There is a container on the piano for the money that each time he practices money goes into. On the day of his lesson, he counts the money and if he has enough for his lesson he is good, but if he comes up short he has to make up the difference. Boy have we heard a difference in how often he practices.