@Darian How long do you expect each illustration will take you? Your craftsmanship here is very good!
To be honest, $500 for 10 images of this quality seems rather painfully low to me, although it could still be worth it for the experience and to work with the author if you really like them and their work. I have also done things for a very low price before if I loved the project and expected it to “take me towards the mountain” in my career.
To share my own experience doing some black-and-white illustrations, I did this piece for a client two-and-a-half years ago, and was paid $200 for it. After doing 9 more hidden picture puzzles for that price, and getting much better at it, the price was raised to $400, and now we’ve agreed to raise it $50 per every 6 puzzles that I do with at cap at $600. It’s also work for hire, but I love working for this client. Time-wise, these usually take me between 14-20 hours consistently. 