Over the years, I've seen lots of conversations about money wasted on homeschool materials. There are entire threads devoted to this on homeschool forums. Each year, a thread will be started asking about what didn't work for people that year, in other words "what did you waste money on this year?" I've always been rather proud of the fact that I have wasted very little money on homeschool materials.
Our first year or two, I mostly bought workbooks, like the Spectrum workbooks, some stuff from local teacher stores, and materials for whatever projects/activities/experiments I had decided we'd do. Then, I found Rainbow Resource, homeschool forums, and tons of reviews of homeschool curricula.
The Spectrum workbooks and other similar workbooks were not a good fit for us.They were not challenging, and had too much review. The first Latin program we tried for Dea was a flop. We didn't like it. Luckily, it wasn't an expensive program. In 4th, I bought her all of the Key to Series. Once we looked at the Algebra & Geometry, we realized they were not enough for high school level courses. They were really nothing more than Pre-Algebra & Pre-Geometry, so we didn't use them. Again, they weren't really expensive. I bought Teaching Textbooks Algebra before Dea had finished TT Pre-Algebra. The thought here, of course, was that she would finish Pre-Algebra & move to the Algebra. She had liked the TT Pre-Algebra when she started it. By the time she finished it, though, she hated it. So TT Algebra was wasted money. So far, that is the most expensive mistake in materials we've made - one level of TT. I don't consider TT Pre-Algebra to be wasted money, even though she hated it by the end, because she did finish it. I only consider it wasted money if it was not finished.
I figure that we have wasted less than $500 total, in our 8 years of homeschooling. That's pretty damn good, especially considering so many have mentioned wasting more than that in a single year. Even if you add up the cost of everything we didn't like, even if we did finish using it, it still comes to less than $1000 total.
I'm very picky about what materials I choose. I do tons of research, look for other options, read reviews, read descriptions, look at samples (when available), etc. I'm very critical about homeschool materials. Over the years, I've pretty much perfected my way of choosing materials. Occasionally, though, I do second guess myself and revisit materials I had previously written off. Sometimes, I'll even ask at forums about the curriculum in question, trying to see if there is something I'm missing, something that would make it worth trying. I don't think there has yet been a time when I have changed my mind and purchased something I had previously vetoed. I'm pretty confident in my choices, and actually looking over how little money has been wasted over the years makes me even more confident.