When the Pencil came out there was a bunch of neat coloring apps. All free or cheap purchases IIRC. I kept a folder of them, but didn’t use any of them over the long term. New Pencil, and I figured I would give them another look. From my folder of apps, the apps either didn’t survive or went subscription based (and not cheap). Not something I want a subscription for, but would pay for a one time purchase.
I was looking for the same thing a year or so ago, and ran into the same problem. Either they’re cheap and require IAPs for more pages, or they just have a flat-rate sub that’s like $70/year or something that seems a touch crazy.
I’ve looked for the same thing for my kids and always was disappointed with how much they want for the apps. I think there is a Disney one on the Arcade but don’t quote me on that. I ended up just finding coloring pages online and saving them as PDFs and putting them in Notes.
I’m not sure if this works for you, but I had the same issue for my kids when they were younger. So, I went searching for coloring pages and then just downloaded and added them to GoodNotes notebooks. Rolled my own coloring books form them. I found plenty of material freely available.
I’ve been using Lake, but it is one of the ones that has become horrible expensive. My go to now is Zen: coloring book for adults which is $8.99 one-time, and $9.99 for the Mac (in case you have a Wacom or the aforementioned Astropad Slate)
Late to the game, but if anyone is still looking for a cheaper coloring app for iPad, I can recommend Zenful Coloring for Adults. The yearly plan is ~$44.99, but there is a 30% discount for new users, so you end up paying $31.49 for the first year. You can also try the app for free for 7 days.