What is the minimum necessary amount of art for an rpg rulebook? I know that (with the notable exception of the original "little black book" edition of Traveller) rpg rulebooks have traditionally included a lot of illustrations, and that certain types of products (bestiaries, adventures with scene-setting illustrations intended to be shown to the players, products aimed at beginners who aren't necessarily familiar with the genre and its tropes) need illustrations, but is this also true for a "standard" rulebook aimed at experienced players containing "crunch" info on classes, spells, magic items, and procedural stuff? Other than making the book more attractive to look at, are "filler" illustrations that just exist to set a mood (and fill white space) and aren't depicting anything practical really needed?
This isn't just idle navel-gazing, since I'm working on a product for publication that I am not capable of illustrating myself (at least up to a standard I would consider acceptable) so I'm trying to gauge how much art I'm going to end up having to buy (or beg) for this thing. A cover illustration is surely necessary, but what about the interior? RPG audiences are accustomed to seeing an illustration at least every 3-4 pages because that's what we've been given for the past 45 years, but if a book doesn't include that would it necessarily be seen as incomplete and unprofessional? Do I need little pictures of adventurer-types rappelling on cliffs in my section on wilderness adventuring, or a guy strumming a lute in the section on bard spells? And if I do need to include some interior illustrations, what is the minimum acceptable amount? Would 3 or 4 illustrations in a 128 page book be sufficient or if I'm going to have that few would I be just as well off not having any at all?
On the one hand I'd obviously prefer not to sink all of my potential dozens of dollars of profit (and more) into art, and I especially don't want to feel obligated to buy or accept a bunch of low-quality art because it's the only stuff I can afford, but on the other hand I think the text of this book is of high quasi-professional quality, and I'd like the presentation to be at a similar level if possible. If I'm going to expect people to pay real money for this then they should feel like their money was well spent, and I'm trying to get a sense of how important interior illustrations are to that determination.
I'm wondering how my 12 (!) readers feel about this. Given that a lot of good art isn't an option, what are folks' preferences between (a) no interior art at all; (b) a couple-three pieces of interior art; or (c) the book must be fully illustrated, even if the illustrations are of amateur quality?