Across the country it is between $8000 to $24,000 a year per student. The median across the country is $12,000. In my homeschool, without overhead, we can educate our kids for about $500 a year for curriculum and maybe another $400 for field trips. That includes co-op costs and piano for the older two. Of course we offset the cost of field trips with fundraiser etc.
One of the reasons your costs are so low is that you are not including a percentage of your mortgage/rent, heating/AC, transportation, food, insurance, computers, the cost of your teaching, (including your retirement plan, social security, and medicare), required classes for your staff, and the opportunity cost of your lost wages
Plus I would imagine you do not have children who need special care such as speech, OT, PT, Communication devices, and/or specialized teaching due to autism, learning disability, and/or intellectual disability.
I would imagine your costs are still lower than those of the public school, but when you look at what the honest costs are, it will much greater than $500