Interesting question, though it's certainly also one I doubt is going to be at all easy to quantify on a national or global scale.
I can answer, however, with respect to London in the Victorian period, by citing some work I published a few years ago on the Toshers. This was the name given to a group of people who earned their living by (illegally) entering the city's sewers and (often by plunging up to their shoulders in the accumulated effluvia and excrement) retrieving material that had been washed off the streets, into the gutters, and thus into the sewer networks. A significant portion of this bounty was made up of small change (and sometimes large-denomination coins) dropped by London's pedestrians.
We know something of this group, their working habits, and their ability to make a living in such difficult and dangerous circumstances thanks largely to the work of Henry Mayhew, a mid-Victorian journalist and pioneer statistician who set out to interview representatives of each distinct group of working-class Londoners able to live, thrive and survive in the Victorian metropolis, publishing his results in the four volumes of London Labour and the London Poor. From my essay on the Toshers, originally written for the Smithsonian but available in its entirety from my website, here:
They were mostly celebrated, nonetheless, for the living that the sewers gave them, which was enough to support a tribe of around 200 men–each of them known only by his nickname: Lanky Bill, Long Tom, One-eyed George, Short-armed Jack. The toshers earned a decent living; according to Mayhew’s informants, an average of six shillings a day–an amount equivalent to about $50 today. It was sufficient to rank them among the aristocracy of the working class–and, as the astonished writer noted, “at this rate, the property recovered from the sewers of London would have amounted to no less than £20,000 [today $3.3 million] per annum.”
The toshers’ work was dangerous, however, and–after 1840, when it was made illegal to enter the sewer network without express permission, and a £5 reward was offered to anyone who informed on them–it was also secretive, done mostly at night by lantern light. “They won’t let us in to work the shores,” one sewer-hunter complained, “as there’s a little danger. They fears as how we’ll get suffocated, but they don’t care if we get starved!”
Quite how the members of the profession kept their work a secret is something of a puzzle, for Mayhew makes it clear that their dress was highly distinctive. “These toshers,” he wrote,
may be seen, especially on the Surrey side of the Thames, habited in long greasy velveteen coats, furnished with pockets of vast capacity, and their nether limbs encased in dirty canvas trousers, and any old slops of shoes… [They] provide themselves, in addition, with a canvas apron, which they tie round them, and a dark lantern similar to a policeman’s; this they strap before them on the right breast, in such a manner that on removing the shade, the bull’s eye throws the light straight forward when they are in an erect position… but when they stoop, it throws the light directly under them so that they can distinctly see any object at their feet. They carry a bag on their back, and in their left hand a pole about seven or eight feet long, one one end of which there is a large iron hoe.
This hoe was the vital tool of the sewer hunters’ trade. On the river, it sometimes saved their lives, for “should they, as often happens, even to the most experienced, sink in some quagmire, they immediately throw out the long pole armed with the hoe, and with it seizing hold of any object within reach, are thereby enabled to draw themselves out.” In the sewers, the hoe was invaluable for digging into the accumulated muck in search of the buried scraps that could be cleaned and sold.
Knowing where to find the most valuable pieces of detritus was vital, and most toshers worked in gangs of three or four, led by a veteran who was frequently somewhere between 60 and 80 years old. These men knew the secret locations of the cracks that lay submerged beneath the surface of the sewer-waters, and it was there that cash frequently lodged. “Sometimes,” Mayhew wrote, “they dive their arm down to the elbow in the mud and filth and bring up shillings, sixpences, half-crowns, and occasionally half-sovereigns and sovereigns. They always find these the coins standing edge uppermost between the bricks in the bottom, where the mortar has been worn away.”
The remainder of the essay deals with other aspects of the Toshers' lives and folklore, but is equally fascinating, I hope.
Sources for all this:
Peter Ackroyd. London Under. London: Vintage, 2012; Michele Allen. Cleansing the City: Sanitary Geographies in Victorian London. Athens [OH]: Ohio University Press, 2008; Thomas Boyle. Black Swine in the Sewers of Hampstead: Beneath the Surface of Victorian Sensationalism. London: Viking, 1989; Stephen Halliday. The Great Stink of London: Sir Joseph Bazelgette and the Cleansing of the Victorian Metropolis. Stroud: Sutton Publishing, 1999; ‘A London Antiquary’. A Dictionary of Modern Slang, Cant and Vulgar Words… London: John Camden Hotten, 859; Henry Mayhew. London Characters and Crooks. London: Folio, 1996; Liza Picard. Victorian London: The Life of a City, 1840-1870. London: Weidenfeld & Nicolson, 2005; Jennifer Westwood and Jacqueline Simpson. The Lore of the Land: A Guide to England’s Legends, from Spring-Heeled Jack to the Witches of Warboys. London: Penguin, 2005.