It depends on the courses. One major class for a first grader is going to be a couple of hundred dollars full scale curriculum etc.., but some people do it on homemade stuff and go as low as 30 or 40 dollars. High school courses are pretty expensive, but college is about $8000 per semester. You are going to want some more info about homeschooling.
Each State in America has their own laws about homeschooling. Some states are easier than others. Every state has an organization to help you get started. You can meet other parents and swap curriculum etc... Good luck