Thanks a lot guys!
Well that was the thing with this project... it was underpaid - and even so I underquoted myself. But it was fun and good exposure as the commissioned artists got space for an ad in the back of the book

Often it's the author who pays and he gets the funding from the illustration budget of his books. As aviation books have a seriously small market (typically no more than 2000 copies of a title) and the book shops and publishers take the biggest chunk of profits, you can appreciate the authors aren't particularly swimming in money either. So for a good deal it's a labour of love, often a balance between the maximum the author is willing and able afford and the minimum that makes it worthwhile.