It's supposed to take 1-2 tubes per drywall sheet so if you're doing up a room then it works out by the case. How much are you planning to use on the machine?

I've not heard of it being used on metal. That's quite an interesting question to ask.

I'm not saying it's a bad idea, I'd just find it hard to justify that outlay for (in effect) a single tube of greenglue. Unless you plan to sell off the other tubes or you have other soundproofing projects lined up, of course. If you're selling off the single tubes you could probably find a few buyers without too much hassle. I'd have a tube off you!