From the solicit:
Going by the 12 pg average for the backstories for 44 issues plus the 4 pg average for additional story for the first 24 issues + front/back covers, pinups etc. gets to 836 pages. The additional covers for the issues #45-110 plus the 88pp Jeph York mentioned (88 sounds like back covers + pin-ups) still only totals just 990 pages.In 1986, Marvel launched Classic X-Men, a series that reprinted the "All-New, All-Diff erent X-Men" era -with a twist! All-new backup stories fleshed out the 1970s tales and delved deeper into the characters' lives, and new story pages were even inserted into the reprints, expanding on key moments and sowing the seeds for future storylines! Now, all this newly created material is collected in one volume for the first time - complete with comparisons and text articles explaining what was changed, what was added, and why! It's the perfect companion piece to your Uncanny X-Men collection! Friendships are forged, allies are mourned and lives are transformed in these rare tales that dance between the raindrops of some of the most revered comics of all time!
COLLECTING: BACKUP STORIES AND NEW PAGES FROM CLASSIC X-MEN 1-44; MATERIAL FROM MARVEL FANFARE (1982) 60
I'm guessing the additional content will be text intros and essays. Krisis, Raffi or someone else might be able to project more precise totals but at any rate, this book is a must buy for me!