Are you saying that this opportunity cost somehow offsets the difference between the $500 and $9,963 It really does not make up much of a difference. An average homeschooling parent does is homeschooling two kids. Average class size it 24. Divide your $20k by 12 = 1,600. There is still a large deficit difference.

If you want more of a comparison of how wasteful (non efficient) and ineffective public schools are - just compare them to equivalent private schools. They turn out better test scores and spend around 25% that of public schools per student.