Not much to add but for me it's always broken down as the top pages in a particular comic book (Cover, Splash, Hero in costume and battle etc) were the A pages and everything else was B or lower. If you then start cross-comparison across books, artists etc. it gets subjective pretty quick and I guess you end up chalking it up to monetary value to determine "A" pages across the hobby. So yeah agreeing with what others said that it depends on how you define A art and which category you are focusing on/emphasizing when doing so.
Ibzan