Why not individuals
I'd never buy individual NPC writeups.
I've bought a few NPC books that include a variety of materials. When I buy a book, I expect a number that won't get used, and hope a significant number will.
My expectations of an NPC book
I'm couching my expectations in terms of letter sized pages.
I expect a significant NPC writeup to include a full character sheet, ready to play. Print and go. It also needs a 1 page writeup, and either a half-page or whole page illustration. Moreover, it should be PC-legal when possible, so that it can be used as is as an emergency character for when a player suddenly needs one.
Minor NPC's, I expect a short-form (Full stats, in paragraph form, but not a full character sheet, or long-form monster data) box, a quarter or half page illustration, and the rest of the page being description.
Spear-chucker types I expect a 6th page of description, and about a 6th of a page for for illo, and 6th of a page for a data-block in monster-type short data block.
I would expect cover art of decent quality, clear cover fonts, and a preview PDF with chunks of two different characters' write ups in the detailed and minor NPC modes. (Sheet for one, data page for another).
I would prefer to have story hooks and adventure seeds for all the NPCs above spearchucker level. And the spearchuckers should be reasonably tied to making the NPC's useful.
EG: If you have a Gang Leader as a major NPC, his typical gang members should have spearchucker entries, and his lieutenants as minor NPCs.
Where not core to the concept, male and female writeups for the same sheet are a nice touch, as are alternate race conversions.
I would expect, for $5, to get some 32-40 pages.
For comparison, you may want to look at A Cast of Thousands for Arrowflight, or Into the Cosmos for Cosmic Patrol. (Into The Cosmos isn't just a Characters book, but includes full character sheets for everything except mooks...)
As for Fate... the various Fate Games are actually quite different from each other, so I'm concerned that you may be setting too broad a goal to do it justice. For SOTC, for example, such a book would be a potential godsend. For Diaspora, it would likely fall quite flat, as it's going to be a nightmare to work in to the customized setting for each home game.
Legends of Anglierre, Star Blazer Adventures, Dresden Files, and SOTC all can readily make use of NPC books tailored to their individual tweaked systems... but likewise, only SOTC is likely to be allowed to be done of those, and even then, you need to discuss that with the publisher.