It strikes me that very little has been said about the costs to the parents of homeschooling.

Indeed. But the primary reason I haven't mentioned the costs when recommending homeschooling is because the costs are nearer to James_Miller than the benefits. If he's going to overestimate the costs or benefits of homeschooling, it's more likely that he'll overestimate the costs- and so it makes sense to focus on the the benefits.

It also works as a door-in-the-face: well, if you aren't willing to devote full-time employment to your child's education, then surely private school is a bargain?