It's not just half a GCSE grade or a slightly better paid job, though, for lots of people.
In extreme cases, it can be the difference between ANY job or a life on the dole (or in prison), if the child is easily led and the alternative is a crap comp rife with crime, drugs, etc. It can be the difference between a "happy" teenage years against been bullied and making them suicidal.
I agree that, for the "normal" child, in a "normal" school, the difference is probably not worth the investment, but at the extremes, I think it can easily be worth it.
One of my clients is a plasterer. His son was really going off the rails in the town's "crap comp", i.e. truancy, drug taking, gang crime, etc. The thing was that the lad wasn't "bad", he was just getting led astray by the people he got caught up with. After several police interventions, with him getting closer and closer to being locked up, his parents had some serious chats with him, and the lad himself said he hated his lifestyle but felt he couldn't escape it when he was surrounded by his "friends" all day at school. He agreed to moving to a private boarding school that was known for dealing with "troubled" kids - cost his parents a fortune they didn't have, but they borrowed, re-mortgaged, and worked extra hours to fund it. It worked and the lad went on to A levels, Uni, and now has a good, graduate job that has set him for life. In that kind of scenario, it's more than worth it as it got him out of the crime ridden environment and gave him time away, on his own, to actually get himself back on track.