[Amps] Why do tubes cost so much?

[Bill, W6WRT]

I have often wondered why big tubes cost so much. The material in them
isn't all that great and I would think because they are larger they
would be easier to assemble than, say, a 6AU6 which is very small and
intricate but sells for only a few bucks. 

I'm not talking about really massive tubes that take a fork lift to
move, but tubes on the order of an 8877 or the like. 

Anyone know?

73, Bill W6WRT