In general, folks creating maps can be considered as specialized illustrators, and it would be reasonable to expect rates comparable to those of illustrators.

https://www.graphicartistsguild.org/handbook is a good resource for setting pricing, but it just shows things prices van vary tremendously.

For example, for book illustrations, they state the size on page, style and quantity of illustrations will affect the price but is $250 to $1500 per illustration. Small illustrations, 1/2 page and less are in the $50 to $500 range.

Another guideline I read once for "visual artists" was to price per square inch (at a standard DPI, for digital).

I would encourage ALL folks who are creating works on commissions not to undervalue themselves or their work!

Just my opinion.

-Rob A>