Sorry, Russell. just found this.
I don't believe the Pope has anything to do with price, his work is free to all. Just like Pride & Prejudice or Tom Sawyer could be published by anyone today and offered at any price, the document you refer to was priced by the publisher.
The Daughters of Saint Paul, a religious community, has taken on the ministry of printing papal documents in serviceable, plain, soft cover, pamphlet form, at low prices. Presumably they have few overhead costs, as the Sisters who do the work are performing a labor of love with little recompense, they make no profit beyond supporting their community, and the original work is copyright free. They have Charity in Truth on their website for $6.95:
https://store.pauline.org/English/tabid/56/List/0/CategoryID/525/Level/a/Default.aspx
It is also on Amazon in paperback for $6.95, offered by the US Council of Catholic Bishops (USCCB), another non-profit organization.
The work you saw was probably offered by a commercial printing firm like Ignatius Press that has workers who must be paid, and possibly did not think would sell enough copies to justify the typing/ebook publishing, especially since you can get the work free, and if you know how, make your own etext. Ebook publishing is new to Ignatius, probably their prices will fall as they get more familiar with it. And still a great service for those who aren't comfortable doing their own conversion - some people have a fear of, and really never use, their USB connection, don't ask me why.
Although Catholicism is the largest religion in the world, many Catholics do not purchase encyclicals, but receive religious instruction from the pulpit. In other words, encyclicals have small printing runs, and can't compete with best-seller pricing on mass market titles. An exception would be the Catechism of the Catholic Church, released in the 1990's, that was a best seller and eventually was released in sub-$5 mass market editions.
Not an expert in publishing, and probably way more than you need on this topic, just my thoughts . . .
And thanks for great tip, Patricia. I was simply highlight/select/copying to a Word file on my computer and then using Calibre. Your method is so much more professional!