Of all the reasons for or against homeschooling, the supposed "real costs" or "missed-opportunity costs" argument has to be about the second-oldest after the socialization question, and it's just as misleading.
The Deputy Headmistress of The Common Room has posted her current thoughts on this, here and here. It's also worthwhile to go back to her 2005 post here, because the comments are so interesting. I originally posted a response to that one here. (The DHM and I have been friends a long time.)
All I can add, to all that, is this: first, you may save money by homeschooling. It depends on your lifestyle, your curriculum, how many kids you have, how much money you were making or spending before, and so on. As the DHM and others have pointed out, you won't be spending money on extra shoes, band trips, and pizza days either, and you may be saving money related to daycare or other parental work expenses. But most people don't begin to homeschool solely with the intention of saving money. As in, we can't afford to send you to public school any more, so you'll just have to stay home. There are usually other reasons involved in the decision--academic, religious, health reasons, bullying, bad teachers, whatever. So from my admittedly limited economic understanding, this is not something you can approach with a simple comparison of costs.
Second, as far as the actual cost of the actual homeschooling goes--that is, minus the arguments over whether or not the kids' shoes wear out faster, or whether you have lower medical expenses because they're not being coughed and sneezed on by thirty other kids, or how much money you won't have to spend on peanut-free granola bars and juice boxes--the only point that all homeschoolers* can agree with on this, is that we're in control of that cost. If we have money to burn and count a whole lot of things as "school", we can homeschool very expensively. If we're broke, we can scrounge and use freebies. In most cases (see the note below), we are free to decide that this year we will or won't teach a certain subject, will or won't have swimming lessons, will or won't buy a new printer.
Yes, you could put together some kind of an "average" family picture, and say that "most" homeschoolers pay a certain amount for math materials, reading books, computer stuff; or that people who spend a certain amount are more successful at homeschooling than others. But what's the point? A glance through any general homeschooling magazine, or through a week's Carnival of Homeschooling, will show such a diversity of approaches and lifestyles that such comparisons would be meaningless. Even within our own family, every year's expenses are a little different: some years we've just re-used what we had, other years we've needed to buy new materials.
Conclusion? There isn't one, except that, like the socialization question, the "costs" question is just as red a herring.
*"All homeschoolers" meaning all who live where they are free to plan their own work and/or choose their own curriculum provider, rather than being required to teach a set curriculum, buy required books, etc.
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