...and do I mean a LOT. I did the calculations here, but for convenience sake, here they are again.
"With Kindlegem, 10% CDR used to cost 322g using the following calculations. From Ruby Crystal, we can calculate that 180 Health=475 Gold. If you divide Kindlegem's health by that of RC, then you can get a ratio for the inflation. 200 Health/180 Health= 1.1... so multiplying 475g by 1.1 would give one the cost of Kindlegem's health, 527.7g. Subtract this from Kindlegem's base price of 850, and you are left with 322.3 gold, or 322 for simplicity's sake.
Season 3 just changed this calculation with the changes to Fiendish Codex. Judging from Amplifying Tome, 20 AP costs 435g, making the 30 AP found in Fiendish Codex worth about 652g using the above calculations. Now subtract that from Fiendish Codex's price of 820, and suddenly 10% CDR is worth a mere 168 gold! Talk about a price drop! "
So what do you guys think, do I just have no idea how math works or did Riot really just nerf the relative price of CDR in terms of cost efficiency?
CDR has never been at a static price per %. The thing about CDR is that it's priced differently for different roles. Very few APs are going to take Kindlegem (which is normally built by Supports/Tanks), while APs are going are more likely to take Codex (and ADs are probably going to take Brutalizer or Stinger).
There's only four items in the game that provide CDR and don't have any other non-measurable factors to them, and that's Kindlegem, Stinger, Glacial Shroud and now Fiendish Codex. Stinger's CDR is actually completely free at the current price point for attack speed, followed by just 6.7g per % for Shroud and then 16.8g per % for Codex.
Kindlegem is used as the basis for CDR calculations BECAUSE it's the least efficient, at 32.2g per %, in the same way we get our other stats using the least efficient basic item.