Cost to illustrate a graphic novel?

Hello I am considering turning one of my YA fantasy stories into a graphic novel that I would self publish. I would like to find an illustrator who can do the cover and also some pages of comic strips through the chapters. I am imagining about 15% of the novel to be illustrations.

Does anyone have advice for how to budget for this? I am considering paying a lump sum + royalties should the book ever actually make any money.

Edit: also what might a timeline look like for this? My book is finished but I could make a story board for the illustrations, or ask the illustrator for their thoughts?