To be more budget friendly, I don't think all of the pictures should be full color.
The cover wil be color of course.
Only maybe one main color picture per important chapter at the beginning. The rest can be greyscale. A few color background repeated images can be used to fool the reader into believing their is more color in the book than there really is. Icons and mini pictures can be grayscale or black and white. Some B/W images can even be trimmed to where the color of background surrounds the black and white images.
Races can be either. The problem I have with color race pictures is that they are less generic.
For pictures of weapons, there is no need to add color to them.
Even then, larger picture scenes can be cut up into portions and repeated in chunks throughout the chapter and book, zooming in on specific portions of the main picture.
Most emphasis should be on the Char & Arms and Creatures Laws. Mixing Color, B/W, and Grayscale images can be done strategically to mix up the variety. Focus on unique RM monsters. No need to do skeletons, bears, and common stuff.
Spell law needs very few pictures. Treasure law will mostly be charts, and maybe a few pictures of equipment which can also be used in the Char Campaign equipment section. No need to fill the alchemy section with very many pictures. They can be B/W or greyscale.