I want to add my two cents to this discussion.
I am an illustrator, and sometimes my clients ask me to give them a small discount. And my way of giving them that is to reduce the amount of extra work I have to do. For example, I usually suggest working without agreeing on a sketch or without making edits. It turns out that I will do less work, and, accordingly, the final product will be cheaper. this works especially well for clients who have looked at my portfolio before ordering - they know what to expect from me, they know my style, and therefore trust my vision.
Another way to get a discount is for large projects or if you need illustrations on a regular basis. Order all images at once, not one at a time - I personally give discounts for large projects (like more than 10 illustrations in the same style) and many illustrators do too.
Hope my experience helps someone!