We've discussed the differences and pros and cons between center lock and 6 bolt rotors at some length before (a post which is actually one of our most read ever), but why are there rotors at any price between $20 and $90?
As you see in the array of rotors in this photo (pretty freaking arty, too, huh?!?!?), there are both center lock and 6 bolt rotors, and there are one piece and 2 piece rotors. And there are solid rotors and rotors with all sorts of crazy cutouts.
6 bolt rotors can come in one or two piece types. One piece rotors are obviously the easiest and cheapest to make. You cut a flat plate into shape and you're done. A 2 piece rotor has that operation, plus you need to make the carrier core, and then you need to attach the two. So more expensive to make, and therefore more expensive to buy.
2 piece rotors have several benefits. Lighter weight is not among them - they're a bit heavier. The otherwise-the-same Avid one and two pice rotors from this photo have a 30g difference between them (I KNOW - criminal!!). So why would I choose for myself and recommend the two piece? Two piece rotors are stiffer and less likely to warp. The carrier is a stiffer structure than the simple plate and it provides better support to the business end of the rotor. I'm sure at some level there's also a detectable difference in braking power/modulation/things-that-don't-have-established-vocabulary-around-them aspects. Two piece are also better at shedding heat. The aluminum carrier core is a better heat sink than steel plate.
The Ultegra rotor in the photo (the one that looks unlike any of the others) is sort of a special beast. The large fins that you see between the carrier and rotor actually get sandwiched between the rotors two layers of steel. This is designed to be a high speed conduit for heat to get out of the rotor and onto the fins, where it will be dissipated quickly. Really good design, and if you are doing big technical descents, worth having.
Then there are different rotor materials. I'm no metallurgist, but when one rotor says "use with resin pads only" then I am sure that the steel alloy used in that rotor is different, and not as good as the one that allows you to use either resin (aka organic) or metallic (aka sintered) pads. I know I wrote a post about the difference between the two at one point but can't find it. Anyway, metallic pads have more bite, are less likely to suffer brake fade in hard braking, and are generally less prone to noise and last longer, but that comes at some expense to the longevity of your rotor. In these "use with resin pads only" rotors, I guess that their what must be significantly softer metal gets the heck beaten out of it by metallic pads. So despite the lower cost, unless you are just toodling around town on your bike in a way that I think few people are doing with our wheels, the "resin pad only" rotors are a bad look and a false economy.
And that's what I have to say about that.