Precisely what I was thinking! 320 pages? Of what, if not a good amount of class powers?
Well, if you look at the PHB index (at 318 pages - but leave out races, individual classes, paragon paths, epic destines, individual feat descriptions, individual rituals) you are down to roughly 115 pages.
If you look at the DMG and look for the 'crunchy sections" that you would need (skill challenges, treasure parcel charts, hazards/traps/monster creation charts, p42 chart, mounted combat rules, and so on) you end up with another 100 pages (roughly).
That brings us to around 215 pages from PHB1 and DMG1.
Perhaps some more depending on how many examples they want to get into for stuff like poisons, traps, monster templates, etc.
And a couple misc things from DMG2 will probably appear (such as trap creation rules).
The variables would be how many graphics they decide to include, how much white space in formatting, and also (more importantly) how many examples of various things... do they put in a lot of monster templates? do they put in a lot of specific diseases and traps and hazards and unique terrain features, etc.
And also if whether or not they include some methodology behind encounter design rather than just the rules to do it (i.e. do they just provide the rules on what the monster roles are, or do they get into encounter mix templates and what types of encounters work better for what types of party mixes, etc.)
Thus, I can easily envision around 250 pages just by looking at the contents from DMG1 and PHB1. What will account for the other roughly 70 pages, I don't know. (though if the page size is smaller or something given that it's paperback, that might be a little bit of the page count adjustment as well)
Do I have any insider knowledge? No. This is all 100% pure speculation on my part.