So when buying an i5 I am in fact getting much more performance per cost compared to the i7, because Intel decided to make the i7 much more expensive in order to make the i5 seem like it has much better performance per cost?
The Core i3-6320 is about $60-70 cheaper than the i5-6600k, but on applications that don't benefit (sufficiently) from multiple cores, it can perform nearly as well. Anecdotally, friends building PCs always compare i5 and i7 and never consider the i3, but for their real world uses (many games!) it would be just perfect (and the savings could be thrown at the video card where it'd make more of a difference.)
Would i be willing to pay 35$ extra per additional core if I knew I would want to replace the hardware within two years anyway? Questionable. But when hardware lasts half a decade? The premium per core per year becomes irresistably low. The i7-2600k will soon have it's sixth birthday...
However, whether "the core is going to be fully utilized" is another question. If you only ever need 2 cores, it is wasted money, plus, if your focus is on gaming, and your budget requires you to make choices based on right now, the extra $170 can be the difference between getting a Radeon 480 or a Geforce 1070.
(Since I hope my next build lasts another 8 years, I'll probably "pad" my build with an i5... I've fallen into the trap!)
Hopefully somebody who knows more about this than me can jump in and fill in the blanks here.
EDIT: I communicated my point poorly and the comments below mine explain this is much clearer terms
Really it is just a way to do differential pricing. Some people will pay $500, some only $100 while the part only cost $25 to make. The way to maximize profits it sell to both, and find a way to justify the price difference.
As a hypothetical example (I'm not 100% sure if this is how Intel does things), and i5 could just be an i7 except it couldn't handle the higher clock rate. So instead of throwing away a chip that's limited in clock... Intel will sell the chip as an i5.
Or perhaps in another chip, the hyperthreading portion got borked because of a piece of dust landed in the lithography. So instead of selling it as an i7, the hyperthreading portion is fused off and the chip is sold as an i5.
The IBM-Cell in the PS3 was a different approach to the same problem. The Cell was specified as 7-SPUs and 1-CPU, but in actuality... the manufacturing process made 8-SPUs and 1-CPU.
IBM assumed that one of the SPUs would fail during manufacturing, so they only promised to deliver 7 of them per PS3.
You can literally see the 8 SPUs: https://upload.wikimedia.org/wikipedia/commons/e/e0/Cell-Pro...
There's lots of ways to approach the problem. Manufacturing is an imperfect process: not everything is made identically. So the manufacturers have to figure out how to test for differences and... if necessary... sell the same "manufactured" chip in different bins.
Phenom.
https://en.wikipedia.org/wiki/AMD_Phenom
https://en.wikipedia.org/wiki/Phenom_II
Not all the hardware of Cell on PS3 was available if you run the official Linux port. Which was later blatantly removed.