At the end of your school year, what happens to the books? As I've mentioned, you can often not rely on handing down the book you've got to your younger child. Even if you can, what about when they are done with it? Can you sell it? I mean, none of the knowledge you bought it for has been drained away, right? Only one or two children have had a chance to handle it. It still looks practically new! It shouldn't be a problem to get close to what you initially paid when you offer it for sale, right? Alas, this has not been my experience. Most options that I have tried for selling my used text books have produced such poor results that I finally decided to keep those books that I consider useful or interesting for the future, and donate the rest to my favorite used books store. Consider this your warning.
I could go on (and on and on and on...) about my opinion of why this happens, but instead let's focus on the positive. There are ways to homeschool your child on a budget. Many of them are dependent on state requirements, so be sure of your state's homeschooling laws before you make a plan.
Borrow course books from the public school. Most public schools have a plan in place for homeschooling parents to sign out text books for use in their curriculum. Even if you are unable to borrow all of the books in a given year, this option can greatly reduce your costs. Just be aware that you will not receive a course plan or answer key so will have to alter your schedule and grading practices accordingly.
Watch for used books. Someone might as well benefit from the plight of those of us who have purchased text books that we no longer need. You can even run across old workbooks that can be used as reference to create your own worksheets or board work. Costs for used text books are often very low when you compare them to new versions. (Example: The $89.00 English Literature book that was in excellent shape when I sold it brought me a whopping $10.00 on ebay after I caved and offered free shipping. I may actually have paid to get rid of it once you take in packaging materials and gas.) You can even occasionally find them for free. Provided you do not need a certain edition of a certain book to meet your requirements, used books are a very cost effective option no matter what grade you are teaching.
Go in with a friend. If you and another homeschooling family are both planning to use the same curriculum, why not split the cost. This works whether you plan to homeschool your children together in your own co-op, or if one of you will be handing down the curriculum once you are done with it. Make sure that you have a plan in place for reimbursement in the event that a book is damaged beyond use or your situation requires a change of plans.
Consider the GED option instead of a diploma. Although it is (sadly and inappropriately) looked at with less respect by some, the GED is a valid option for high school completion. Depending on your child's future plans, this may allow you more control over your high school student's curriculum choices. Without the requirement for specific text books, you will be able to pursue a variety of free and low cost learning options including used text books, free online content, real life application as an "unschooling" tool, and other creative educational opportunities. Again, be aware of your state's requirements before you make plans. We want everyone to be in compliance while pursuing their best option.
Online diploma programs. (This is what we are doing currently.) While online programs are going to make a dent in your wallet no matter how carefully you choose your school, the variety of programs out there provide an option that will meet just about any homeschooling family's needs and keep you from wondering what to do with all of those text books at the end of the year. Be careful when doing your research. Make sure that the school you choose is properly accredited and that the diploma will be accepted at your child's future college choices.
As for all of those curriculum books you haven't been able to find a new home for yet, why not keep them. You never know when someone in the family might need a little brush up on their skills or when you might run into a family who needs to provide a quality education while on a strict budget. Remember, no matter their resale value (or lack thereof), those books still hold all of the educational value that they did when you first purchased them.