I've been thinking about writing an RPG book for the fantasy world I'm storytelling in, but I'm shite at drawing creatures. I've paid about 10$ each for simple character art portraits before, but what would a somewhat detailed black and white sketch of an animal cost me you think?
Really depends on the artist but if that's what they've asked before that's what I'd count on.
The bigger problem is probably quantity. Ain't much of a bestiary with only one beast in it.
/ic/ would probably be more knowledgeable about this sort of thing than most people here. but at the same time /ic/ is full of assholes so maybe you'll get nothing.
I'm confident however that it will cost much more than you expect if you want any kind of quality.
I'd recommend you don't start looking into the illustrations until you've done a fair bit of the ground work on your bestiary first.
I would more or less finish your beasts and then commission in bulk instead of paying per portrait. Unless you want to go for multiple artists which might be cool variety.
Try something like fiverr maybe
Another factor is that "somewhat" is quite vague. Do you mean simply a sketch or it is a fully drawn image, with lightning and all that?
Also, the amount of details can seriously increase the cost, like asking him to add this and that until you're satisfied.
Deadlines are important.
If your commission ref. document is badly done and you have to correct or specify lots of stuff later, the artist may charge more.
Have a very solid visual idea of the creatures before anything, changing your mind results in extra hours for the artist.
I don't know how prevalent this is, but the artist I'm currently hiring charges the same for color and black&white.
Finally, artists have varying levels of business skills, which may work into your favor.
The key thing is to write out contracts ahead of time so the artists can't Sue you for using their art (which you bought) in a commercial sense. Make sure you credit every artist correctly somewhere in the supplement. Strive for consistency or at least a similar style among all artists. Describe the monster very thoroughly before letting the artist create.