I dont't even know why they both to go to such R&D lengths / expenses, if they're just going to turn around and heavily gimp the chips before they make it to market. I mean, really Intel???.
It shouldn't cost them much, if anything. They design their chips with it in mind. Most of features such as VT, AVX, and HT can be fused off. Clock doesn't even have to be binned anymore since its multiplier locked on the non-K chips. It'll be even easier for the Pentium and Celerons to pass working/not working since you can set the frequency so low.
Sometimes the particular market demands the chip so manufacturers are forced to disable cores just to cater to them. Now Intel seems flexible enough that they can simply cut cores in design and quickly make a new mask with smaller dies. At one point they had 6-7 different dies for the Atoms, and it was more than 5 years ago. If they were individually designing them they would never be able to do that.
So making a new die isn't difficult as long as its based off a common design. You just cut cores/caches out. It's same with their Gen graphics. Every year they talk about modularity. Ring/Mesh has to do with this as well. Crossbar interconnections used in older chips like the Nehalem was not conducive to adding/deleting cores.