I have an easier solution to this and it won't cost you a penny:
1) Subtract one console + games til grades improve
2) If grades still haven't improved, keep removing items til they do or til the room is bare.
3) If the kid still refuses to improve his grades remove all things that make him happy: social event's, car, phone etc....
4) If steps 1-3 fail it's time for a good old fashion passionate @$$-whipping.
5) If the kid still doesn't get with the program then......you have a stupid kid and it's time for and the short bus.
I know there are parents out there who will say the state will take him away from me well in this case the state's your friend and who needs a dumb@$$ kid like this anyway.
(Disclaimer: this post is meant as a joke...well the last statement was but not steps 1-5)