The value of a painting is not known until it is bought by someone. It is hard to decide which is the most expensive painting in the world as the price tag of 100 million USD 100 years ago is different with the same 100 million USD nowadays. So we need to take the inflation into account rather than simply comparing the ordinal prices.

On the other hand, the worlds' most famous paintings such as Monalisa are owned by museums and not for sale, so they don't have actual prices rather than insurance value or estimated prices. Monalisa has an insurance value of $100 million in 1962, which in today's money will be around $ 680 million.

It seems No. 5, 1948 By American artist Jackson Pollock is the most expensive painting in the world. It was sold at $140 million in 2006 (current value - $156.8 million) via a private sale. The new owner of the painting has not revealed identity.