A link-boy was a boy (or young man) who was hired to lead people through the streets of London and other large cities in the time before streetlights were available -- approximately from the early 1600s and as late as the mid-1800s. The link, in this case, was a cheap torch made of pitch and tow.
Some link-boys worked in the employ of thieves and cutthroats, and would lead their charges into a dark alleyway to be relieved of their belongings. Because of this, link-boys had a poor reputation as a group. However, link-boys made most of their living not from guiding gullible visitors through town, but through carrying the smoky and smelly links for those too fine to carry them themselves, and who were not likely to be misled into misadventure. It was also common for a sedan chair (popular in London and Bath in the 1600s) to be accompanied by a link-boy, who would also be charged with finding fares for the chairmen.
"...after supper with the young ladies, bought a link and carried it myself till I met one that would light me home for the link. So he light me home with his own, and then I did give him mine."
-- The Diary of Samuel Pepys, September 10th, 1661
While link-boys were a bit of a luxury, they were certainly not expensive. These were aspiring toshers and pure-finders working a night job, and fees were rock-bottom -- reportedly a farthing a trip, and presumably as high as that only because a smaller coin didn't exist (the half-farthing wasn't introduced until 1844); Pepys' payment of a half-used link may not have been an uncommon trade. By the 1840s the spread of streetlights, an improved police force, and cheap Hansom cabs were putting link-boys out of work.