It depends entirely on the situation, there isn't one single split that works in every scenario and every collaboration.
That being said, based upon the labor involved I would argue (as a person who both writes and illustrates) that the artist should either have a substantial payment upfront or more than 50% of the cut.
The reason for this? A writer always writes. Because actually typing is a rather minor part of the process.
A writer writes in the shower.
A writer writes when they go to the hairdresser.
A writer writes when they take an evening walk.
A writer writes when they go out and meet people, experience things, think about life, when they hug their cat/dog/children. Hell, a writer fucking writes when they have sex.
Because writing is derived from experiences, thoughts, and the occasional spontaneous idea that pops up amidst those things.
This means that even though the writing process is more broadly life encompassing and can take more thought-time to bring together to a script that makes sense, that time is spent doing a wide variety of things that the artist cannot do when tied to their tablet. You simply cannot argue to me that the artist that locks himself up in a lonely room to work on intensifying their carpal tunnel syndrome should get paid equal or less than a writer. Even when all the more labor intensive stuff like editing, spellchecking, research etc is taken into account it rarely matches the labor intensive hours an artist puts in to illustrate those script pages.
For clearer example, most of my drawing sessions consist of me blasting intense music while speeding through one routine after another, taking the occasional break to do dishes or laundry or something else to prevent wrist injury.
Most of my writing sessions consist of... basically turning my tiny apartment into a home spa/relax unit for a few hours. A spa unit that happens to involve my laptop.
Putting these two beside each other, if the writer in me and the artist in me were two different people, I would argue that writer me only deserves 30% of the cut. Because while artist me is rushing to meet deadlines, writer me is getting his nails done while pondering how to word the next sentence to get the right effect.
But that's just me. The easier solution to all of this is just that each and every one decides what sounds fair to them and then go out and match up with people that agree. Or just illustrate their own projects to avoid the hassle.