Re: Is Singapore Math expensive?
Personally, I really recommend the HIGs. Using only the workbooks, or the workbooks and textbooks, it is possible to completely miss the mental math methods that are so important to the curriculum. Also, the curriculum is based on a progression from concrete demonstrations of a concept using manipulatives, to illustrated representations of the concept, to abstract work with the concept. The HIGs have learning activities using manipulatives for most lessons, which I think are necessary for getting the full benefit of the curriculum.
We do not need the Extra Practice books here; they're for kids who need a lot of extra practice with the same types of problems that are in the text and workbooks. We do use the Challenging Word Problems books, but they aren't strictly necessary. We also spent quite a bit on various manipulatives, but many of the manipulatives can be made at home. Some of them are printed in the backs of the HIGs to be copied, cut apart, and used during lessons.
I think Singapore will end up being economical for us, since most of the books are not consumable and we plan to use them with multiple children. It would be a little expensive for one child, although most math programs involve an investment. I find Singapore has a good resale value, so I can always sell later if we don't wear them out first.