The cost of homeschooling is a completely individual thing. A boxed curriculum will usually cost you more money per year. Piecing together your own curriculum can cause a wide variety of costs. There are a number of websites and books that you can find on how to homeschool for less. Additionally, there are tons of websites for printing free worksheets. The library is also an invaluable resource. Basically, you can spend as much or as little as you wish.
Here are two websites where you can look at homeschooling on a budget. Please note that this is general info. I don't claim to agree with everything written.http://homeschooling.gomilpitas.com/weblinks/livelihood.htm
My personal notes: I do think you can homeschool for very little if needed. Since I use the Charlotte Mason method and the curriculum available at Ambleside Online, I have found much of the material is available for free online. Many older literature books are online and in audio form. I have found librivox.org to be very useful in our home.
That being said, I have found that at most I have spent $1000 one year and as little as $100 a different year. This is what I meant earlier when I said that putting together your own curriculum can vary greatly in cost. I will tell you that my most expensive year has been at a homeschool convention. Hah! Those conventions can be traps if you don't go knowing exactly what you want. That one year I didn't. In subsequent years, I've gone to the convention with a specific list in hand and I did much better.
The longer you are at your homeschooling experience the better you will get at assessing your real needs and can go from there. As is common among over achieving mothers, we can be a bit overconfident at how much can be accomplished in one year. You'll get the hang of it with a little practice.