I personally cannot stand it when companies arbitrarily limit a product just to sell it to somebody. It makes no sense to me, if you think about it:
Grado's plastic cans being cheaper than alluminum or wood doesn't necessarily surprise me, and the SR325i has tighter tolerances than teh SR60i. I do not know if plastic is much cheaper than alluminum when buying parts in bulk, I know that machining those parts would be cheaper with plastic. The same tooling you'd use for 1000 of those alluminum cups would go bad at about that 1000 parts, but due to the pliability of plastic, you'd see almost unlimited parts and you wouldn't have to run oil. Less tooling, less wear and tear, and less consumable usage.
Wood is similar, but I think that wood requires more intimate attention, though I have no experiencing machining that. That being said, doesn't Grado press their own shells for the plastic cans? I don't think they do the alluminum or wood parts. I believe based on the pictures that they are brought in from outside, so outsourcing increases cost a bit.
So there's some argument there, you could say it costs more for the higher end cans, plus they cherry pick drivers or at least claim to, for the higher end models. But when a company does what microsoft does with their server operating systems, or even stuff like windows home basic or whatever the hell it is that limits apps running and all that... that's going too far. They had to write additional code to make that do that. Why even sell a product that you're selling for less that you programmed more to even make? That's just ridiculous!
Also, for cheap woodies I'd gladly buy some. My girlfriend has some SR80i with the jmoney headband, and some woodied grado cups would be awesome. one thing I wish more people made was the inside part of the driver housing, the part that holds the driver and the cushion. If you made that in wood, you'd have a corner on the market since nobody makes those. I think a large part of that issue is that it would be prohibitively difficult to install. Thinking about it now that's probably exactly why nobody sells them.