Within very wide limits, the number of pages in the proceedings has almost no effect on the cost per volume.
I don't remember the exact figures, but from memory, after discounts etc the cost per volume is going to be about €40. The kind of variation is about €5 over about 400 pages, e.g. maybe 700 pages is €38 per copy and 1100 is €43 or something. It's also nonmonotonic. The cost per volume can be cheaper by adding pages full of nonsense.