If you live in the Bay Area, you should know about RAFT. RAFT stands for Resource Area for Teaching. It's a pretty wonderful concept. Industries all over the Bay Area donate their manufacturing and office "waste" to RAFT and RAFT sorts it, thinks of great things to do with it, and offers it back to teachers (including homeschoolers) for a very low price.
Things I have found on my visits to RAFT: Klutz books with dinged edges or dried up glue for $4/each, reams of 11 by 18 paper for $5, RAFT assembled science kits with great directions for $2-6, zippers for 10 cents each (we used these to make number lines!), textbooks and chapter books for 50 cents, headphones by the bagful for $5, yarn for 50 cents a skein, sturdy tape dispensers for $1, huge sheets of tagboard for 10 cents, bags of odd shaped fabric scraps or cardboard pieces for $3, and lots more! I've gone out of RAFT with a shopping cart loaded to the top for less than $50.
They also have a green room with lots of die cuts, a large laminator, etc. for free or at cost use.
It is a membership based ($50/yr), and children under the age of 12 aren't allowed on the floor. I usually go with another mom and there's a grassy area outside with a picnic table that we take turns watching the kids at while the other mom shops inside. Usually only one of us has a membership and we divide our stuff off the ticket later (they are also nice about giving us a subtotal part way through).
Even if you don't go, check out their activity sheets which offer lots of great ideas for using "throwaway" materials to teach tons of topics in all subject areas.
If you are thinking of starting a co-op or looking for inexpensive resources for homeschooling, look into RAFT! I would suggest calling ahead and asking what kind of documentation they want you to bring to show that you are a homeschooler (they are a non-profit and have to document that all their members are teachers at public or private schools). They have asked for different things on different years so find out their current policy for homeschoolers before you go.