"Despite his fame, Martial did not actual publish anything until more than 15 years after his arrival in Rome - around 80 CE. But he certainly gave "readings" of his poems, either in private houses or in public venues like theaters and lecture halls (a feature of the better bath houses). It is also likely manuscripts circulated privately, much as did the "sugared sonnets" of some guy named Will during Queen Elizabeth's I reign.
On the other hand there was a thriving book trade, which is not what you would expect in a society where all books had to be written by hand and probably less than 20 % of the people could read. Becoming a publisher was surprisingly easy. You didn't need the metal type or printing presses which made book and newspaper publishing so cumbersome until the advent of our digital cyber age. All you needed was a bunch of scribes - mostly slaves - and have someone read out a book. Each of the scribes would write down what the reader dictated and hey presto! you had more books. The major expense was papyrus - not cheap - and literate slaves also cost quite a lot. So we hear that books were expensive.
Although the initial investment and materials to be a publisher was a bit costly, the logistics and inventory handling was pretty simple. Book publishers were also the book sellers, and the books were produced right in the shop itself. So you could have five to ten literate slaves, and you'd end up with five to ten copies of whatever best seller was then in vogue. In the shop - called a taberna - the books were on display up front, and the slaves would be writing more books in the back. When Martial's first book went on sale, it costs five denarii.
Was five denarii expensive? Well, to get a feel for the costs we need to look into ancient Roman coinage and prices. The coinage system in Martial's time was essentially that of Augustus. The emperor had decreed that 1 gold aureus was equal to 25 denarii which was in turn equal to 100 sesterces which was convertible to 400 asses (a bronze coin, not the animal, and the singular is as). So with this rate of exchange, Martial's book cost 20 sestercii or 80 asses.
You'll find some books that put a sesterce into modern dollar equivalents. But since Rome didn't have built-in system of inflation like ours, the equivalences soon become outdated. It's better to see what the necessities and luxuries of life actually cost. Still, even then the relative worth of goods and services in antiquity cannot be equated to those today and we have to fish around to get an idea of what 20 sestercii means today.
For instance, what did it cost for a day's food? In Pompeii - which was buried by Vesuvius when Martial was about 40 - a visitor recorded the cost of his consumables at an average of 4 sestercii a day. The diet was also by no means extravagant. On a typical day you ate bread, cheese, and had wine to drink. When you splurged you might have a sausage and some vegetables. In other words, Martial's book cost 5 times what you needed for a a day's modest diet.
Another way to look at the cost is to compare it to the income of various occupations. We know that a Roman soldier earned about 1000 sesterces a year and so he would have to set aside a week's wages to buy Martial's book. This agrees pretty well with the costs of our visitor to Pompeii (who did spend money on more than just food). But paying out a week's expenses for a book is not something your average Roman could easily afford.
So who bought the books? Obviously those with more money than a soldier. The poet Juvenal, who as a younger man probably practiced law (for which you were not allowed to accept fees), said that it was possible for a single man to live "comfortably" on the income from investments of 400,000 a year. Now if he got a return of 5 % - pretty typical for the time - he would get 20,000 sesterces a year. That must have been a comfort indeed and the 400,000 in assets actually defines the equestrian class. So Juvenal's comfortable single equestrian gentleman made about 55 sesterces a day. He could have easily afforded Martial's book.
Finally, we need to point out that a book in ancient times were scrolls and contained not much more than a single chapter of our own single volume codices. If you bought Martial's complete works you'd have to buy 15 "books" which would have run to 300 sesterces. That would be over five days pay for Juvenal and over three months pay for our Roman soldier. Today we can buy Martial's complete works with English translations - double the size of the originals - for about $40. So yes, books were more expensive than today and not something that your average Roman would buy.
Did Martial make money from his books? Not much. He might get a one time payment in a deal with a publisher or patron for a new book, but there was no copyright. So once the book was on the market, anyone with a backroom full of slaves could make and sell his own editions. So throughout his life, Martial had to rely on handouts - sorry, that's "patronage" - from his rich friends."
From
The Life and Epigrams of Martial - A Most Merry and Illustrated History
His references include...
Martial, Loeb Classical Library
Latin: An Introductory Course Based on Ancient Authors, Frederic Wheelock, 3rd Edition, Barnes and Noble, 1976. A number of Martial's epigrams are in the exercises.
Martial: The Unexpected Classic - A Literary and Historical Study, J. P. Sullivan Cambridge University Press, 1991. Informative but expensive.
Bathing in Public in the Roman World, Garrett Fagan, University of Michigan Press, 1999. The first section uses the poetry of Martial to learn about the Roman baths.
A Day in the Life of Ancient Rome: Daily Life, Mysteries, and Curiosities, Alberto Angela, Europa Editions, 2009, A general "daily life" book. One chapter discusses costs of goods during Trajan's time and which were similar to those during Martial's lifetime.
"The Display of Elephants in Ancient Roman Arenas", Jo-Ann Shelton, ISAZ Newsletter, No. 21, pp. 2 - 6, (May, 2001)
Experiencing Rome: A Visual Exploration of Antiquity's Greatest Empire, Stephen Tuck, The Teaching Company.
History of Rome, Garrett Fagan, The Teaching Company.