In PDF, the single volume for ease of searching. I do essentially all my game preparation, as player or GM, on the laptop, so PDF is my primary purchase these days.
If hardcopy, two volumes, because binding an 800-page book so that it lasts is expensive. The pricing you list in the question suggests that the binding for the single volume won't be sufficiently robust.
Even in PDF form I might prefer smaller volumes divided by subject, as particularly large, dense documents can start taking frustratingly long to page or search through - but PDF splitting software can solve that.
I can tell you from nearly 30 years of selling RPG books... the vast majority of people will go for two smaller books.. If you can make them stand on their own merits each (as slightly different products, not quite one book split in half) then even more people will buy them in that format, by buying one first and the other later (when they would just never buy the larger more expensive book, ever).
On the other hand, if you have an established hit, you can THEN fuse two smaller books into one larger book and have another group of people buy that product. But that's another story.
I'm a big fan of Into the Unknown's old school 5 booklet approach, so you definately get the 2 books answer from me.
If it's separated in pdf format i'm good with that too, as long as the book separations make sense.
Give me one book with character options and rules to play, another for running, one for magic, one for items, and one for monsters and I'm happy.
I can share the player book with the players. If they are playing a spellcaster they can look over the spellbook. If not they can ignore it. As i'm running i know they don't need to see the gm section.
I love the modularity of separate monster item and spellbooks, but have never seen a system that fully integrates it into the structure. For example i'd like a monster book just for demons and demon kind, and a separate one for fey/shadowfell, etc.
That's going off topic though. I know that all these can be done in a single book with chapters, but it's easier to organise in my mind as separate focused books.
I'd pay more for multiple books. Inevitably, there's always one section of the book you're going to reference more. If I can get that as a separate book I'd do it. That way I can have a travel copy and a home copy. As a DM, I always buy two Monster Manuals for this reason.