Well, it all depends really. You can make it as expensive or as cheap as you like. There are so many free opportunities these days that it really needn't cost the earth. Of course if you decide that you really must have the lastest items from the educational supply catalogues; this year's must have gadget; the very best artist supplies etc etc it could end up costing you more than it would to send your child to Eton. It doesn't have to though, and doing it cost effectively doesn't mean skimping and giving your child a second rate education.
Home educators tend to be a rather canny bunch, and asking for ideas or helpful links on one of the free to access email groups will give you more than enough food for thought! You don't even have to pay money to join an organisation. There are plenty of free ones that will give you just as much (if not more) help and support and the freely available Home EdBlog rings are also an incredible source of ideas. Of course if you're reading this you've got access to one of the greatest sources of information and ideas going - the internet. Google is a mighty tool if used well.
Of course another great saving you can make when you home educate is on holidays. Many home edders become seasoned campers. Even if camping isn't your thing, holidaying out of the school holidays is a much more economical option. Before we had children we always went on holiday in September, and we've been able to carry on doing that thanks to home education. It's quieter, cheaper and touch wood we've always been very fortunate with the weather!
Necessity is the mother of invention or so they say, and I'd say that it's certainly a great educational driving force.
Interestingly, research shows that home educated children from poorer backgrounds - ie the working classes, do better than the children of the more affluent - ie middle classes and up. Interesting that isn't it?