I think it depends on what you are looking for.
If you have clever DC then you would expect them to achieve their academic potential whatever school they attend, state or private. If DS came home with poor grades when he could achieve better, yes, I would be unhappy with him, but not because we pay for his education.
It also depends on what type of private school you choose. If you pick a highly selective, top 20 school, results are obviously one of your key motivators for choosing private. If you pick a more all-round, less pressurised environment, then perhaps, along with a fantastic education, you are looking for breadth of experience, sports, activities and value these just as much as results.
I've never once felt let down by DS, either at his old state primary, his prep or his current senior school, but he does tend to do everything quite wholeheartedly.
If that changes in the future, I won't think "what a waste of money", but I will still put a rocket up his arse, because he will be letting himself down.