1. How many books/booklets and how much total page count do you like in a box set?
2. What extras do you like to have on top of the booklets?
3. Are you willing to pay more, per page of content, for a box set than for a single book? If so, why?

I'm not sure if I can come up with an exact page count or cost per page count, but I see that a boxed set has several potential advantages.
(1) Everything can be kept in one place, so more smaller items can be included.
(2) Because of #1, rules can be divided into Player books and GM books so that a player doesn't have access to secret stuff.
(3) Boxed sets have space for lots of maps.

I think those are the things I like best about boxed sets.

finarvyn has answered for me.

I wouldn't want dice, unless the game has special dice that are not normally available. Character sheets might be useful.

1. I'm a big fan of the 3 booklet system if having basic intro rules, gm book, and an advanced "going forward book"

2. Dice required for the game and handouts and play aids that are useful beyond whatever introductory adventure is shoved in.
With few exceptions I'm not a fan of intro adventures but that's a personal hang up.

3. Only if it's neatly put together and aesthetically pleasing enough to show off especially to kids and teenagers who I try to indoctrinate.

1. As much as relevant. Though I prefer a general page count of 50+ total. Ideally two to three booklets of 25 - 50 pages each.
2. MAPS! MOAR MAPS! Even artsy fartsy maps! (and also cut outs, distributable props, and other goodies. and maps! )
3. Yes... within reason. Because cats like boxes. But truthfully, it's all about ease of transport of pamphlets (less in the gaming bag, faster reference, split DM v Player info,) and MAPS! Did I mention maps yet?

Don't go crazy on the booklets. It does let you do the GM/player split, which is a nice, and maybe a separate adventure. But a half a dozen books are a pain to manage. Ideally, keep them slim enough to be saddle stitched (stapled).

Another vote for the extras. Maps yes, but all kinds of handouts are great. Custom character sheets for different classes, player backgrounds, props like stained maps or pages, art, ship blueprints, etc. Put some thought and effort into it; avoid filler. And don't worry about some of the traditional things, like dice or pencils. Nobody who plays an RPG needs dice, and unless you're named Wizards of the Coast it's unlikely you're targeting brand new players.

1. 3 books is great, 2 is fine, 1 is acceptable if it's big (but in that case, you'd better have something else along with it to justify a boxed set).
2. Maps!  Maps maps maps. Player handouts are good, I like dice too.
3. I'm willing to pay more for a boxed set because boxed sets tend to come with neat extra shit (like maps, dice, so on).  Everything in one place is handy as well.

1) 2 books (GM + players') is good. 3 can also be good if the third contains info organized in such a way as to be immediately useful for the GM (while the second is "long term" GM info).
2) Maps, or props which are "permanently" useful (i.e. not something like a character sheet which I can just print or anything related to a specific adventure). Player aids would be great too (basic turn structure, other info which doesn't fit on the sheets)
3) Not necessarily. Depends on execution. I like boxes as a rule, but they need to have stuff which I find useful.

My answers:
1. I like to have 2-3 booklets with not more than ~200 pages total. Too much and I feel like a big book has been cut up and put into a box.
2. I like character sheets, maps, and dice, especially custom dice. Also cheat sheets and play aids. I like the feeling of "wait, there is more" even if it comes down to a flyer or a "what's in this box?" sheet.
3. I'm willing to pay more for a box set. It would be nice these days if PDFs were thrown in, though. Dungeon Fantasy was a $60 box and then you'll pay another $40 for the PDFs.

Enough different booklets so that premium space--inside of the front cover, last page, inside of the back cover, and outside of the back cover--all manage to get used for things are needed in play the most, whatever that is for the system.  If characters get made a lot, some of it can be used for that.  Also, enough books that 6 to 8 people can easily make characters at the same time, with minimal wait.  It's possible to do a fairly decent job of this with as little as 3 books, if the nominal "GM book" has some of the equipment in it.  Mainly, though, I want to have at least 3 back covers that would make a passable GM screen.

As far as extras, I can take or leave dice.  I've got plenty, and don't care for specialty dice.  The rest, I'm 50/50 on.  Maps are great, but since I'd prefer to do my own setting and adventures, they are or somewhat limited utility.  A bunch of 2-page (one page front and back) locations would be nice, though.

People have already covered what I want in the box, but I also want to state that the box itself needs to be sturdy enough to stay together after some time on the shelf. If I can use the box lid as a die rolling tray (better with more shallow boxes), so much the better. I do not want weird ass plastic designer boxes that inflate the price by a lot and make it harder to fit them on bookshelves.

I like boxed sets for RPGs to have:

Decent maps. This is really a great opportunity to show off the game world and is nice to lay it out as appropriate to players.
Nice if it's in colour or a well drawn black and white map.

I would like the rules to be split into separate physical books,
Such as a Player book, GM book, Tech/Magic book, monster book etc.. (as appropriate to the setting)
 
Nice to see those inserts they used to have in Boxed sets advertising their other products.
I guess less necessary these days what with having the Internet now, but still it was fun to read those old product booklets.

I don't know if anyone has said this but empty space. Because I like to keep follow up books in the box too.

Tom

Don't care for boxed sets anymore since the days of TSR buying out SPI. Like starter sets even less.

• A rule book and a setting book. Players IME never read anything so I don't really need separate GM and Player books. Perhaps add a book or booklet with a couple of scenarios, and generically usable prefab characters and NPCs.
• Maps and floorplans! Preferrably generically usable ones, not specifically tied in to a scenario.
• I like a reference sheet/GM screen. Perhaps also add a bunch of reference sheets specifically for players.
• No need for dice and pencils!