We spend between $300-$500 per year for two children, dd 6 1/2 and ds8. To avoid a huge oh-my-goodness moment in August, I buy our books yearround so that it's more affordable. I have already started purchasing books for next year. We also ask for a lot of books to be given as birthday and Christmas gifts from grandparents. I also highly recommend Paperback Swap. I haven't been on in a while because none of my books were being requested, but I was able to get several very good resources for the cost of shipping some of my own resources. I'm to the point now where I'm trying to save books for my grandchildren (isn't that crazy?!?!) since my dd has already declared that she wants to homeschool her children one day. Of course, I realize she'll probably change her mind 100 times between now and then, but I would like to have some nice things to pass down to her if and when she's ready.
If your child doesn't like going to the library, perhaps you could request books from them online or by calling ahead. Then when your dh is home you could pick them up. That's just a suggestion to make things a little more cost effective for you. This past year was the first year we were able to buy everything we needed and haven't had to make trips to the library out of necessity. We have checked out leisure reading books for ds8, who is a voracious reader. I think we would go broke if we had to buy books for him weekly to satisfy his appetite!
We also have an iPad, and MANY CM friendly books are free on iBooks. I know for a fact that all of Arthur Scott Bailey's Tale of... books are free and many by Thornton Burgess are free as well. Wind in the Willows, The Complete Winnie-the-Pooh, and other literature recommendations are free too.
I like to compare prices on several different sites when I'm ordering. The ones I look at first are: http://www.abebooks.com, http://www.half.com, and http://www.amazon.com. Abe Books frequently has books we need in a good, but used, condition for less than $5 with free shipping.
If you have a family member or friend who also homeschools, consider borrowing and swapping from each other. I am able to do this with my sister-in-law, who is also using SCM for her children. I was able to loan her our Module 1, Pollyanna, and some Shakespeare for her to use this year. I haven't borrowed anything from her yet, because we're one year ahead, but I'm sure I will someday!
Hope this helps,