Ref to your point:- "here is a lot more variation in the materials and techniques used to make tyres than to make a solid disc of metal." - Actually no, there's a lot more alloys of steel than kinds of tyre setup in the world. That's just steel, let alone Aluminium and the rest. Even considering tyres are also made up of nylon beads, steel wire banding, anti noise membranes and such.
I'd be honest here if you know metallurgy you should know about heat treatment at the very least. Let alone the differences in source material compounds.
- Harder metal so lasts longer against same pads
- Tougher so less chance of it cracking
- treated so lower rate of oxidation, again lasting longer
- machined to better tolerances and quality checked more so you can be sure they're the same time and time again.
All in all, if the opinion is they're initially just all identical cuts from the same piece of cheese, it couldn't be furthre from the truth I'm afraid.
Generally all depends what you wanna do with your discs. I spent what more on mine recently than I've ever done for a set on a car, mostly cause of the car it is. I still wouldn't use them if tomorrow I was drafted in to drive it in even a 20-lap race.
I'm only piping up here because there's 3 things I avoid saving extra on for my cars - brakes, tyres and steering. All you've got in the way of hope of avoiding the unthinkables.