Actually, there’s tons of free content out there. The biggest problem is trying to figure out what is the best fit for your child’s learning style, and appropriate content for the topics to be studied.
For example, Khan Academy has a ton of videos/courses, and is always adding more (Pixar in a box is really interesting!). You can take many free college level courses on Coursera and other MOOCs, or directly from individual universities.
Last time I checked, most library content is free (okay, just teasing). Actually, our local library makes thousands of titles free to download. They also have dozens of free courses each month (mostly oriented toward techie stuff … but not always).
Our daughter has basically taught herself to do computer graphics and animation just by watching YouTube videos, and texting with other young artists online. (She turned down my offer to enroll her in animation classes at a local college). She also watched a ton of excellent science videos on YouTube (for example, see Vsauce).
Brainpop has a bunch of free educational videos.
Most areas have homeschool co-op groups, where parents share teaching responsibilities.
Our local school district makes the entire school curriculum available via virtual school, at no cost to homeschooled students.
And the list goes on, and on and on …
I could fill several pages with free resources. As I said before, the biggest problem is trying to figure out which ones to use.