Short answers, pulled from the story.
The standard price was one-and-a-half guineas, which equalled one pound, eleven shillings, and sixpence, or ten shillings and sixpence per individual volume. This price held unchanged for nearly seventy years, from around 1821 until 1894.
The Edinburgh publisher Archibald Constable established the format. The first three-volume novel at the standard price was Walter Scott's historical novel Kenilworth, published in 1821.
In 1894, both Mudie's circulating library and W. H. Smith issued circulars announcing they would only pay four shillings per volume rather than the previous price. Production collapsed almost immediately, falling from a hundred and eighty-four titles in 1894 to just four in 1897.
Charles Edward Mudie bought novels from publishers at five shillings per volume, less than half the retail price. He charged subscribers one guinea a year to borrow one volume at a time, or two guineas a year to borrow four volumes at once.
A standard three-volume novel ran to around nine hundred pages and an average of a hundred and sixty-eight thousand words across forty-five chapters, with a total word range of a hundred and fifty thousand to two hundred thousand words.
The Lord of the Rings is a three-volume novel rather than a trilogy. Tolkien originally intended it as the first of a two-work set paired with The Silmarillion, but his publisher rejected that plan. For economic reasons it was published in three volumes between the 29th of July 1954 and the 20th of October 1955.