The thing is Transformers are not famous for using metal.
So when mp-1 was launched it was a huge eye opener even to non transformers fans. The binaltech series just made things of what I would expect from the masterpiece line.

Note, I didn't care for all other transformers toy line as it is expected to be not as premium as the mps. When sideswipe came out, it was really like "oh is that it? All plastic?" It took a while for me to sink in, lower my expectations by quite a bit before I started to enjoy the masterpieces again.

Yes cost is a factor, but this is masterpiece we are talking about.