A highway robbery is an expression, that refers to the selling of a product or service for a blatantly large price, exploiting the lack of competition and alternatives.
The idiom comes from the fact, that a “highway robbery” required a lot of courage and determination, as it proved to be the most dangerous, but most profitable encounter, a bandit could have found himself in.
The expression originates from the last decades of the 18th century, first recorded around 1780 and 1790.
The purpose of “highway robbery” was to imply the audacity and the blatancy of the asked price and to use the expression’s power to make the seller reconsider his morals and the price he’s asking for.
“Highway robbery” turned truly actual after the explosion of the automotive industry, as highways started to link remote parts of the world together.
It is a globally known phenomenon, that petrol stations and resting places on the highway are the modern day descendants of “highway robbers” as due to a lack of competition, they are asking for intolerably high prices.
Urban Dictionary has a list of entries on the subject, written and uploaded since 2006.