It's not necessarily that expensive of a machine that you need to make them, but once the machine is set to make them you can't really make anything else without pulling out some of the tools (if you have a more expensive machine that holds more tools it would help, but then you have to make that much more money with the machine to pay for it). To complicate the matter further, you don't really need a live tooled CNC lathe (a live tooled lathe costs about twice as much as a standard 2-axis lathe (the live tooling servos can be bloody expensive as wel)) to make many other items in the genre of parts Mod Quad sells.
What I am guessing they do is make the impellers in a two setup operation on a vertical CNC mill. The problems you have with this method is that you have a harder time controlling concentricity and roundness.
If the Mod Quad impellers are in fact aluminum (I still haven't confirmed that), you have to wonder why they made that material choice. It's not the worst choice, but it seems like the delta in cost between the aluminum and stainless wouldn't be a compelling enough reason to make the decision based purely off that.
Like I said before, if you have a problem with a Mod Quad part, you need to call them. They have some of the best customer service around. :thumbsup: