Glad you're getting some helpful advice! In terms of money-saving, probably you know about the following already, but I wish I had!
(1) When you buy books, buy them used on amazon. Sometimes our library doesn't have things, esp. textbooks, and so I buy them used. "Acceptable" quality is usually more than fine for us, and I can buy, say, a $100 textbook for $5. I haven't looked for the Usborne books, but I'll bet you can find those for far less than the bookstore (new) prices.
(2) Homeschool Buyer's Co-op website. They have a lot of stuff I wouldn't use, but if you've independently identified some kind of software or other program, they often have group discounts. My kids liked BrainPop (web program that has little educational movies), for instance, and the Co-op had a discount.
(3) EPGY Open Enrollment. If this still exists, it's a great deal, compared to the regular EPGY. You don't get a teacher or a transcript/grade, but you get full access to the software, which at the elementary level is all we wanted. The software tells you how the student is doing, but you just don't get the official EPGY grade record.
