I use cotton, poly and cotton/poly for piecing but try to use the same type throughout. And yes, it is expensive. That's why I hunt for estate sales that have sewing listed in the description. I've purchased huge bags of thread for $5 and it's rare that I've come across brittle thread that needs to be tossed out. As long as it doesn't show through, I use any color at all. I know that's probably a big old taboo, but it's thrifty. Interestingly, at a lot of estate sales, I've seen bits of thread wrapped around cardboard or whatever -- talk about thrifty! Those, sad to say, get tossed.