The lower grade the book, the less impact fading would have.
For example, let's say you have a 9.0 book but it's faded. I could see 6.5-7.0 tops
Let's say you have a 6.0 book but it's faded. I would say 4.0-4.5 tops
If you have a 3.0 book that is faded, well, I think you would be around the same, maybe bumped to a 2.5
Does that make any sense? I don't know if that holds true for CGC but that is kinda how I set it up in my mind. Personally, I would rather the copy you have that a tattered 2.0 copy. However, I would expect to pay 3.0-3.5 range prices for your copy, even if structurally it is higher.