Good for you. But I think you are over valuing your work and the impact it has. Is this going to revolutionize things for your students that so far have managed fine without this work? What about those students that go to different universities and may never see or learn of this? I doubt this work will have the impact you think it will have. There are a lot of academics publishing books. That kind of is what they do. It matters. But mostly only to a small audience.
Your publisher is involved because they don't have to do a lot of work or take a lot of risk. Editorial and layout work is a something they'll recover on the first few hundred copies and they are very good at that because that's their core business. It's a fixed cost. And since you bring the students, they'll have a strong guarantee those copies will get sold. Everything after that is profit. Most publishing deals leave paying the author as an after thought. They pay themselves first. And unless you are a best selling author, that means they pocket most of that 50$.
If you sell 1000 copies, that's 50000 dollars. That's basically the editing and layout cost more than covered. You may have spent years of your life writing and learning. But editing and typesetting that is a lot less billable hours than that. Weeks or months at best. Possibly just days.
So, from the publisher's point of view this is an amazing deal. You do most of the work, including bringing an customer base (i.e. your students for years to come), and they get most of the profit. That's why academic publishing is such a hot business.
And let's be fair. You are compensated by the institution you work for (presumably); probably pretty well even. Having a book with your name on it helps enhance your value and status. Especially if it sells modestly well. That's why you get the big money. Publishing and being good at that is part of the job as an academic. You are already being payed for that. This isn't you going above and beyond what you are supposed to be doing but you doing your job.