Eustice Tilley, the dandyish mascot of The New Yorker, telegraphs his estimable class status with a number of chic-yet-dignified sartorial choices, a top hat and topcoat among them. But the accessory that most efficiently communicates Mr. Tilley's caste is his monocle: a single, bespoke lens secured round his neck by a fine string and held aloft before his discerning eye. By the time Tilley graced The New Yorker's inaugural cover in 1925, the image of the bemonocled man-about-town had already become a thing of caricature. How did the monocle become a symbol of wealth?
It was a symbol of wealth from the start. The standard monocle is essentially a small magnifying glass without a handle (though early versions generally had one). The monocle can be helpful in reading small print and before the advent of modern refractive-error testing it was thought to be capable of correcting myopia, but sporting one as a general part of one's attire was always something of a fashionable affectation. Like the lorgnette, spyglass, and, a direct ancestor, the quizzing glass, the monocle basically originated as a faddish accessory of those with the cash and the inclination to purchase such things. It was most popular with the moneyed classes in Europe in the 1820s and '30s, and experienced a revival in the 1890s.
h/t invisible furry hand