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Just out of curiosity I looked at a book of fiction promoted in Shameless.
It is of 178 pages and it costs £9.22 (ex VAT) that would make it cost £11.16 (inc VAT). In my view rather a lot for black & white paperback of only 178 pages which do not actually get to a story until page 9.
But not only that, it seems to have 2" margins all around in a 15.24cm wide x 22.86cm tall book, so as you can imagine there are not many words per page. (None of which are Justified. It also has page numbers on every page and First Line Indent is huge).
What I am hinting at here is one surely has to give value for money? Even books of fiction have to compete on price. Even more so if one is not famous!
Take a look at these sample prices >> https://www.amazon.co.uk/s/ref=lp_266239_nr_n_8?fst=as%3Aoff&rh=n%3A266239%2Cn%3A%211025612%2Cn%3A72&bbn=1025612&ie=UTF8&qid=1488473704&rnid=1025612
Granted few if any are using expensive POD, but that only means that when using POD one cannot add a high royalty or the price just looks really out of place amongst the above examples. What you set as royalty may not look much to you, and no doubt those sample books may even be making more for the publishers, but due to POD being expensive to start with you cannot expect to make the same profit as mass-printed books, and actually sell many or any.