I'm actually a writing coach and editor-for-hire, but my job involves a lot of copyediting, so this post is illuminating, especially this advice.
When clients ask if they're going to be paid enough for the essay we're working on to cover the costs of working with me, I usually say, "Probably not. But when you work with me, you're learning a set of writing skills that will make ALL of your writing more valuable going forward.."
I love your suggestion of pitching this as an investment they're making. That's good advice, and I'm going with it!
THANKS