Or, depending on your and your artist's preferences, you can go by an hourly rate. I suggest $50 an hour.
Artists: Don't forget to add sales tax if your region requires you to charge it! You don't want to be on the hook for that at the end of the year.
From my research, it seems that a fair rate for a roughed, pencilled, inked, and coloured page is $250.
An Ambition Series-like book contains:
11 chapters with 2 cartoon summary pages
(11 x 2 x $250) = $5,500
+ 1 team image = $150
+ 1 back cover image = $100
+ Cover image (reworked chapter image) = $100
+ Graphical overview (summary, reworked chapter images) = $150
There's a vehement distaste in the creative community for "spec" work, or being asked to work for free with the promise of "exposure". (I'm from Canada; we learn early that you can die from exposure!)
There are "name-and-shame" bulletin boards about companies that solicit unpaid artwork or repurpose artists' without permission. So it's best to treat this like any other commercial transaction: Just buy what you need.
I recommend setting up a "kill fee" agreement: If the project goes off the rails for whatever reason (cancellation, dissatisfaction with the artwork, etc), the artist will be paid 50% of the agreed set fee, or for the hours they've invested so far.
It's simplest for everyone if the artist is engaged under a "work for hire" contract. This means they agree to be paid once and transfer all the final assets to the entrepreneur, along with the rights to those assets.
The entrepreneur then holds the copyright for the artwork. They're free to reuse it however they need (promoting the book, using images with articles based on the book, etc.) without paying any future royalties to the artist.
The trick is paying well and fairly up-front so both parties feel it's a good deal! (Again, the hope is that things go so well that the artist makes money from more collaborations, rather than relying on fees for reproduction rights.)
These are my recommendations -- but the working relationship between the artist and entrepreneur is up to you!