On what subject/supply do you spend the least money? And, again, why? Is it unimportant? Or is it just because you can hand it down? Is it something that people (parents, inlaws, another hsing family) give you consistently?
Science! I got a bunch of really cool kits given to me by a homeschooling mom with older kids who was moving and didn't want to take them with her. We do those sometimes, but otherwise for science we study a topic at a time - get library books about it, find videos (Magic School Bus, documentaries, etc) to watch on the topic. Other than maybe some crafting supplies or printing a few pages here or there, we spend nothing on science right now.