In my mother's day publishers would take the publication on, advance the author a reasonable sum on account of anticipated royalties but then only pay a small fraction of the publication's selling price to the author by way of royalties.
Today, the whole process has been made rather more difficult in some ways, for example publishers probably faced by so many requests to publish, tend mainly to accept works from agents who of course take fees for their services. Then there need to be web sites for the author's work to be advertised upon, added to which, the likes of Amazon will offer to sell on-line digital versions of the publication.
Registration with the British Library and equivalent libraries, for example in New York, is best also left to the publisher in my view.
Paying a publisher upfront to publish a work enables the publication to proceed whilst at the same time providing the author with a far higher percentage of the sale price as royalties than is usually the case for the traditional publishing route. Indeed the higher royalties might even in due course, cover the publication costs.
One difficulty is that even the paying up-front publishing route, is not always available.
None the less so far at least, I have found a publisher of the kind described above, more than helpful. Indeed several weeks after submitting my draft, a second publisher came forward to offer to publish but by then I had selected a well reviewed publisher, which has just produced a draft cover for the book, publication of which is expected in about April 2020.
The publisher's artist has designed the intended book's cover from my fifty year old photograph of Cappadocia rocks, a draft of which looks thus: