You raise a good point. I did some research to see the general percentage of public education costs are dedicated to payroll. According to [0], payroll + capital + debt service accounts for 84.2% of education costs in Texas, which are costs that homeschoolers cover in foregone salary, home costs, and mortgage interest.
The home costs and mortgage interest need to be paid anyway, so it's really only the opportunity cost of the parent. On top of that there are employment costs the homeschooling parent will avoid by not holding down a job - things like transportation and wardrobe.
That said, most homeschoolers I know spend a lot of time and money making sure their children have opportunities to socialize with other kids, which is something you get for free with public schooling.
[0] http://www.fastexas.org/study/exec/exhibits/ex7.php