Its been awile since I reviewed my info on grading but don't remember the "carcass price equivalent index" term you refer to. I have a copy of the USDA grading standards and other stuff which I could review if you could expand on the term a little more. I assume the YG/# you refer to is the yield grade, Yes?
We sell to Tyson/IBP and the base price given to us is for a choice YG3 carcass. Anything different than that will get premiums/discount. They pay a premium for YG2's.