I'm enjoying reading this thread and feeling a bit vindicated. A while back we had a pretty contentious thread on this subject. Honestly, it is part if why I don't get on the forum as much as I used to.
People will pay what they will. Like any collectible, and these yellows are collectibles, condition is important. I don't know that these will always retain their value (bbq is enjoying popularity right now). Because of that I would not think of them as an investment. Pay what you like, recognizing that if you ever part ways you may or may not get your money back.
Lately I've settled into enjoying my modest collection. I continue to be amazed by how much untapped potential there is in these simple kettles. They really are great.