Gpa uses comicconnect, and the best way to track the values of higher end books is to watch the auctions of heritage , comiclink, and comicconnect, as far as 'auction' prices go , 'retail' asking price normally lands 10-20% above that to give room for negotiations. Sometimes it can be a pain in the keeping up with all of that, but I enjoy it
also once books get into higher $ numbers , it's not as though they are traded/sold on a daily basis, and often when they do sell, it is via private sale .... and most of those aren't reported to gpa