I guess the way we've looked at it is everyone in the family has a job ... and the expectation in any job is that you apply yourself 110% to the task and strive to excel. His job is school, then sports. Our expectation isn't for straight A's, a gaggle of homeruns, or a Perfect Game ... it's simply work really hard, every day. He knows that his work ethic, GPA, BA, and ERA are his keys to the next level, so he decides how far he wants to go.
He's the one that has to look back at this era of his life 20 years from now and decide if he made the most of his opportunities and abilities. It won't matter to me if I'm watching him play ten years from now in an adult league at the local park or on TV ... it's his life, not mine.
Since baseball is his passion, he sets his own goals, he decides how hard he wants to work towards them. We've made it clear that if he wasn't 'working' at some sport endeavor, that he'd be required to work a 'real' job with all that available free time to help pay for things like gas, car insurance, etc. ... i.e., I'm happy to subsidize the costs of his sports and privileges as long as he's applying himself to something productive with a goal in mind. If he wasn't playing baseball but instead sat around the house IM'ing his friends nights & weekends ... then he'd be working to pay for his own transportation.
He does get a cash 'bonus' for A's each semester. We've used the lever of he loses the privilege of recreational transportation (dates) if he doesn't achieve a minimum GPA (i.e., he can still drive himself to his 'job' ... games and practices). But we've found that his desire to play baseball at the next level is a bigger incentive than his car keys to focus on his school work.
The Big Question? How do I get him to clean his room?