Cite specific examples (individual drawings or entire books) as a starting point.
How are they similar to and/or different from what you need?
Writing, illustration, editing, and self-publishing for comics and prose.
Content ranges from all-ages to young-adult; secular and liberal.
In a product for sale, or a gift for display? Of a book: how many editions or ISBNs? Who will possess the original artwork?
By default, as a contractor using his own supplies/facilities (rather than being an employee of yours), Charles will retain COPYRIGHT of his commissioned work; he will LICENSE it to you, according to the project parameters. A purchase/transfer of full copyright will be contingent upon mutual agreement.
An editor must declare a text finished and proofread, before art production in earnest.
An art director must ensure good continuity and provide necessary reference material.
A publisher must decide the printing/distribution method, provide technical specifications accordingly, and finance the project.
If on a strictly flat-fee basis for commissioning and/or publication rights (license): 10s or 100s of dollars for a single piece of artwork, or anywhere from 100s to 10,000s for an entire graphic novel or novella/novel of artwork. Purchase/transfer of full copyright may command higher than these metrics.
Viable alternatives or supplements to flat-fee may include: royalties, advance-against-royalties, copies of final product for gifting/vending purposes, etc.
Higher compensation warrants a tighter deadline and more exacting art direction; lower compensation allows Charles a more leisurely pace and greater creative freedom.
Any work which he might do for you would take him away from his own projects. Make it worth his time, effort, and resources.
He will value your project as much as you do, and will prioritize it according to your commitment.