The corollary to the "single book"/"affordable and small" request is that the core of the game has to be slimmed down very significantly.
Taking 4e as a model, simply by cutting the level-range to just the Heroic tier cuts out sufficient powers, magic items, and monsters that it should be possible to compress to a single book. And there are savings that can be made by reducing font size and eliminating whitespace.
Or, alternately, look at the BECMI "Rules Cyclopedia".
A single-book version of D&D is certainly possible. Whether such an approach would be desirable (either for WotC or for the players) is rather more debatable.
(It's also worth noting, of course, that this refers only to the core of the game. The door should absolutely be left open for supplements adding any and all options that people care to have - and that's true whether the core is one, three, five, or fifty books.)
And do you really think just the "core" of the game is all what most people want?
I don't believe that for a second.
If the only way you print the "core" of the game is in one book and then put all the "modules" in other books... most modules of which most players will consider an important part of their respective games... then you've gained nothing. Each of those players will still be buying multiple books to reach a point where their game seems complete. All you've succeeded in doing is making it easy for the BECMI-type players to "one-stop-shop". But how many people who are going to buy this game are BECMI-type players? I would submit that they are very few.
Now, if WotC is fiscally able to print the game multiple different ways, then yeah, that's great for everybody. I myself was one of the many who said last year during the whole "beginner's box" debate that it might very well behoove WotC to release the product in two different methods-- the traditional hardcover three book model of the entire game split into PH, DMG, MM... and the three boxed-set (or Essentials-style small book if you'd prefer) player/DM combined product by each individual Tier. But then again... this was before the whole concept of "modules" were revealed-- modules that you'd need to include in each Tier boxed set, otherwise you're asking players to buy parts of the game upwards of six times, let alone three.
But if the choice on opening day is to buy the game in the traditional 3 book format of PH/DMG/MM or in a new 3 book Core/Modules/High Levels format... I think they'll be less likely to have players go ballistic and will actually make more money if they stick with the traditional one.