Paine's Common Sense belongs to a genre of pamphlet literature which is today almost extinct but was very important for the dissemination of information and culture in the Early Modern period.
Broadly speaking, cultural historians define a pamphlet as any unbound publications that is not a periodical and which is generally between 6 and 48 pages.
Pamphlets were cheaper than books, and often cost about the same as a newpaper. A 16-page octavo pamphlet could be cheaply printed on a single sheet of large paper, making it the ideal format for cheaply conveying political or religious ideas. However, not all pamphlet literature was so serious. Many many pamplets were produced with songs and ballads, often with new and bawdy lyrics set to a familiar tune.
The pamphlet functioned a piece of long-form journalism, in a time when the types of partisan periodicals that today publish such opionions, did not yet exist.
In terms of a market, people bought pamphlets because they provided a cheap way to get ideas, news, and opinions, often in a longer and more detailed way than could profitably be covered by a Newspaper. Also, pamphlets could be published cheaply and more or less anonymously, which made them ideal for publishing scurrilous rumors and radical political notions.
In terms of cost, most pamphlets in Britain and the United States in the later 18th century cost around 1-2 shillings. To put this information in context, at that time there were 12 pennies to the shilling, and a loaf of bread cost about 2 pennies, and a newspaper about the same. One shilling (12 cents) would buy you a meal (with drink) at a tavern, or about a pound of cheese. So not nothing, but something that the ordinary working person could afford fairly easily. I would hazard that 24 cents in 1775 would be very roughly equivalent in purchasing power to about $15 today, to give you another basis of comparison.
Pamphlet literature remained part of the information ecosystem into the earliest years of the 20th century, when it declined as it was supplanted by modern periodical-style weekly and monthly newsmagazines, for the most part. By the 1890s, paper and printing was cheap enough that it was economically feasible for even relatively poorly funded political and social movements to publish and distribute pamphlets.
While it is true that initially there was an agreement with Robert Bell, the Philadelphia publisher who first published the pamphlet, it fell through after the first run sold out and Bell claimed there were no profits. I'm not clear on whether this was in fact true or not, but it led to their falling out and Paine eventually sought other publishers, at one point he paid out of pocket for a run of 6000, and eventually just turned over the copyright to anyone and everyone who would incur the costs of printing themselves.
It was never true that Thomas Paine endeavored to make personal profit from the sale, as he fully intended to donate any profits from the first edition to the acquisition of supplies for General Montgomery; then encamped in Quebec.^1
He did try his hand at profitable bridge building, which didn't go terribly well, and eventually led him to France and his involvement in the French Revolution.
1 Gimbel, Richard (1956), A Bibliographical Check List of Common Sense, With an Account of Its Publication, New Haven, CT: Yale University Press, Thomas Paine, "The [American] Crisis No. VII", Pennsylvania Packet