Have you ever met a sharecropper? If you’ve been in a taxi in Boston, or almost any major US city, the answer is yes. Like tenant farmers in the post-Civil War South, the typical modern cabbie pays dearly for the right to do his job. Taxi medallions - the government-issued licenses that let a car be used to transport passengers for hire - are insanely expensive: The going rate for one in Boston is about $500,000. Most would-be cabbies thus have little choice but to rent a medallion from somebody who owns one, paying through the nose for the privilege of working long shifts at low wages, in a job with high expenses, no paid vacation, and little prospect of ever being in business for themselves.