Originally posted by Crumplepunch
The majority of professional illustrators I know work in colour and then have their work greyscaled for black and white books. Certainly I have seen colour originals of artworks that went into B&W Exalted books of earlier editions.
I expect the cost of a colour book is in the production phase, not commissioning.
Depending on the artist, a color commission can be double to four times more than black and white. For printing in b&w vs color, it's much the same, but then we're talking about multiplying thousands of dollars.