First grade looks like it will be the most expensive. I also figured that even if we are not homeschooling at the time, or doing something else, they are still good reference materials. We have a great library system, so we'll get as much of the reading material there as possible. Also for some books, you can often find good substitutions for less money, although TWTM suggestions are all fairly reasonable. Also, the DP World History Encyclopedia is an excellent substitution for the Usborne book.
Some people prefer it.
I hope this helps. Besides seeing the possibility of giving my kids an outstanding and personalized education, I love it's affordability. We'll probably use Singapore Math. First grade looks like it will be the most expensive. I also figured that even if we are not homeschooling at the time, or doing something else, they are still good reference materials.