In November 2017, which artwork set a record as the most expensive sold at auction?

Leonardo da Vinci's Salvator Mundi sold for $450 million in 2017. The paintings by the masters are almost exclusively held in art museums and rarely come up for auction making the demand astronaumical. It is not unheard of for paintings to sell for over $100 million.

In 1962 the Mona Lisa, which is considered the most valuable, was estimated to be worth about $100 million. In today's money that is closer to $1 billion. However, if it did go up for auction it would probably sell for much more than that. It is priceless.

https://en.wikipedia.org/wiki/List_of_most_expensive_paintings