That's great, Simon. You might even make a profit, but regardless, at $40, you're not risking big money. I've occasionally seen newsletter ads for middle-grade books. Do these work t all?
I tried one newsletter ad for MG about 8 or 9 years ago. I can't rememember who it was with, but I know it was a disaster. I've been angling for a BB for the past 2-3 years, but honestly I only apply with the MG novel when I've worn out my welcome with my adult books.
MG ebooks are a waste of time for me. Since I switched to advertising them heavily in paperback they've done a lot better. (over 400 copies in the first half of this year, just over 250 for all of last year, and only 100 for all of 2019.)
Unfortunately, POD pricing doesn't compare with mass produced paperbacks, and if you bring the retail price down to something more-or-less acceptable there's no margin for advertising.
To be honest my ongoing, non-stop efforts with my MG books are really just me being stubborn. But there's this little voice telling me these are really good books to get kids reading scifi, and if the series does hit the spot one day, I'm sure it'll go ballistic.
Sheer stubbornness is what gets us anywhere.
Depending on where your market is for the paperbacks, it might be more profitable to use a job printer and then send Amazon the books to warehouse and mail as sales are made. Your margin could be better and I know some people do that. But it's another learning curve, and job printers by and large in the U.S. are crooks or incompetents (take your pick) who routinely overcharge. Years ago, we literally found the asking price cut in half when we were about to end our relationship with one printer because we could not afford to pay what we had been. They had that much margin built in; that is, they saw we had no way to compare prices and so they plucked a nice fat one out of thin air and gouged us mercilessly until we said we had to end it. Thus, I am not exactly recommending that route, but I have heard it can result in a higher per copy profit. So can hand selling at events, if you don't count the value of your time and all the other associated costs. Been there, done that, too.
Another possibility is a licensing deal with a trad publisher who has distribution clout, such as Scholastic, but again, that's a learning curve, and you are a writer first. Maybe your agent could look around for a publisher to license them?
Anyway, for now, I'm sure the BB will get you some fun results.