Taxi medallion

A taxi medallion, also known as a CPNC (Certificate of Public Necessity and Convenience), is a transferable permit in the United States allowing a taxicab driver to operate. Several major cities in the US use these in their taxi licensing systems, including New York City, Boston, Cambridge, Chicago, Philadelphia, and San Francisco.
The medallion system is a government-created intentional constraint on the supply of taxicabs, and since historically cities have increased the number of medallions much more slowly than the growth in demand for taxis, medallions have generally been considered a great investment; though recently the increased supply of cars for hire created by ridesharing companies has been eroding the value of medallions.
Historical prices.
Boston.
In 2013, the price of a medallion was $625,000. In 2014, that price had risen to around $700,000. Shortly thereafter however, medallion prices began to sharply decline. According to the Boston Herald, by the end of 2018, the value of a Boston Tax Medallion had collapsed; after reaching an all time high in 2014, prices had fallen to around $40,000, representing a 95% loss.
Cambridge.
In 2014, a medallion was worth nearly $700,000.
Chicago.
The total value of all medallions and assets related to them had a value of $2.5 billion in Chicago in 2013. 
In 2012, medallion prices ranged from $87,000 to $385,000. In 2015, Chicago's average medallion price was under $230,000, down 30% from 2014. In 2018, they had dropped to a range of $30,000 to $100,000.
New York City.
The total value of all medallions and assets related to them had a value of $16.6 billion in New York City in 2013.
In 1962, the market value of a medallion was around $25,000. The price rose steadily. In 2005, an individual medallion was around $325,000 while a corporate medallion was approximately $375,000. Around 2010, the market value of a medallion was around $600,000. It peaked around 2013 at over $1,000,000. Between 2014 and 2015, New York City's non-corporate medallion price dropped 45%. In 2015, the price had fallen to approximately $650,000. As of 2018, one can now purchase a medallion for less than $200,000. On July 11, 2019 sixteen medallions were offered at auction, three of which sold for $137,000, $136,000 and $138,000, while another thirteen medallions had no bidders. Since many cab drivers took out loans in order to afford medallions when values were high, many have subsequently been forced to declare bankruptcy. In September 2020, Marblegate Management LLC, the largest holder of NYC medallion loans, decided to write off $70 million in debt indigent cab drivers still owed.
Philadelphia.
In 2005, the value of a medallion was roughly $65,000 to $80,000. As of 2018, the value was around $50,000.
San Francisco.
Prices were once $250,000 but have since declined.