The brake rotors you buy today are softer than the ones available years ago and will not last as long. The metals are softer but this is an advantage in that it will allow better grip of the bake pad when it is braking pressure is applied. Many brake pads today are ceramic an will last 3 times as long as the old organic or metallic pads. So while the materials have changed and some parts will not last as long and other parts last much longer, the prices have come down on parts and braking efficiency has improved.
An engineer once told me a perfectly designed part is one that fails at the end of its expected service life, so if a car was designed to last 5 years, the thing would self destruct at the end of 5 years. Now that of course, doesn't really happen, but manufacturers are getting better at not overengineering parts, so that is why some new design parts don't last as long. They wouldn't be able to sell new cars if they didn't break down at some point.
Like everything else on cars, braking efficiency has improved, but in my experience, prices have not come down on parts. Sure you can buy cheap rotors for $50 and pads for $25, but you won't like them because the rotors will warp and the pads will be so hard you'll lose braking performance. Good quality parts still cost money, and the newer and more expensive the car, the more the parts cost. A friend had to change the front brakes on a Cadillac CTS, and if memory serves me correctly, parts alone were $600.
As far as rotor warp, I have no doubt that manufacturers have been changing the metal composition over the years, but some car companies are able to provide rotors that don't chronically warp. In the 90's GM used to use the same braking system on a car that was 500 lbs heavier than what it was originally designed for. Those puny rotors on a full size Cadillac used to warp all the time. GM has been increasing brake size over the years, and now a 2007 Buick Allure has brakes comparable in size to a 1986 Corvette. GM however, is still behind.
I recently worked on a Mazda 3 that had obvious rotor wear, but they weren't warped. Why? Because FoMoCo gave the car rotors with a large enough diameter and width to take the heat. The rear rotors weren't vented, but they were twice the thickness of a Pontiac Grand Am.
Generally, I don't have rotors turned on my GM cars because if they have warped at their original thickness, they are more likely to warp after they've been machined. I take the money I saved in labour doing it myself and put it towards new rotors.
