I think the declared value only comes into play for shipping and insurance purposes when cgc sends it back to you (and obviously for tier submission level & cost/turnaround time)
You can declare it with any value you want. Cgc reserves the right to upcharge you and put the book in a higher submission tier, though I don't think that occurs often from discussions I've seen.
As far as what ur book could be worth, it's a question of how much damage could be done to the book and how much it could downgrade it. For ex, I have an X-Men 94 I bought 35 years ago which I just found out had some color touch, but the opinion I rvcd was that it would significantly damage the book to remove it, so I decided to leave it as is.