It's because there is a lot charge and they can get a certain amount of pieces out of the piece of aluminum they use. One or six would have been the same price. If I did a larger run they would be even cheaper. And they are quite hefty too.
I may redo the knobs at a later date after it's assembled. Right now they are 1.75" in diameter and will protrude 1.125" from the surface of the faceplate. I might want them to protude only about 0.9", I haven't quite decided yet. I'll need to see it assembled.