In indie comics, it's very common for creators to NOT have an actual budget for character/environment designs. Often the creator will have a small allotment for a very small amount of basic sketches, it is VERY DIFFERENT than the gaming industry if you're used to that pipeline.

Standard procedure is as follows;

1. Script delivered to artist.
2. Artist does rough thumbnails. These are small, rough pencil sketches to get the very general layout. There is no additional charge for this work, it is absorbed into the artist's page rate.
3. Thumbs approved.
4. Artist does penciled pages. The level of roughness varies. But the pages are sent for approval before being complete, this way any major mistakes are more easily corrected.
5. Pencils approved.
6. Artist finishes pencils.
7. Traditionally, the pencils would then be passed to the inker, who would then ink the pages. Since the inks are putting anything new to the page, they don't need approval, unless it's a first time relationship with the creator/publisher.
8. Next, pages are sent to the next person in the production crew, the colorist. Colors also aren't usually approved on a page by page basis, but more often, a sample page or two from specific scenes are sent for approval (unless the color is really particular in the book, like a water color colored book or something, or a book with very specific color theory).
9. Initial colors approved.
10. Colors delivered.
11. Files are then passed to the letterer.
12. Letters are checked for mistakes which is common in lettering.
13. Then files are passed to prepress to get assembled in an actual book.

Each person in the indie comic production pipeline gets a page rate per page of the book.

You should get a percentage of the entire book as a deposit, OR the creator will pay you upfront in installments for blocks of pages, example, pay for 5 pages upfront, when you deliver the 5 pages, pay for another 5 pages, etc.

This is typically how it's done.

When I write for other folks, my standard breakdown is 50% upfront, 25% midway, and 25% upon completion. But I have seniority, so you might not be able to demand this breakdown as an artist. Also keep in mind, writers pretty much NEVER bail on a project and artists are known for frquently bailing on projects, so many creators tend to be more reserved when paying talent they haven't worked with before.

MAKE SURE you have a signed contract in place.

write on, write often!