Ooooh I got the situation twisted. I thought you were the one hiring the assistant and ask how much you should pay her.
Honestly it's hard to tell, maybe I see it wrong but you do basically half of the job right ? That leaves her with the storyboard, sketches and lineart (+ writing if she's the writer as well).
That sounds like a lot of work to do for 40$ a day.
The best solution would be to always set your rate atleast at the minimum wage but realistically, most people/artist who hired artists can't afford that much (or won't).
You take the risk to pass on the opportunity if you ask for too much and like I said above, an assistant will most likely work until the comic is done so you also pass on a steady income.
It's not the best answer but when setting my rate I always go for how much time I invest but also how much "bullshit" I can take before I complain or feel uncomfortable
If 40$ a day don't sit you right, maybe 80$, maybe 150$ etc ? you have to figure it out your threshold.