Expensive does not equal better, and neither does inexpensive equal inferior. I’ve used crappy yarn that was expensive and I’ve found really good yarn I like working with that was a bargain.
You have to use the best quality you can afford. If you don’t like working with a type of yarn, you’re less inclined to finish your project. I have UFO’s that became that way because I hated the yarn I was working with.
I’m one of those people who buy yarn with specific project in mind, but I always by extra so I don’t run out. A majority of my stash is made up of extra yarn left over from projects. What it really comes down to is trying to get the biggest bang for your buck. I do a lot of shopping around on the Internet to find the biggest bang for my buck.