Isleworth Mona Lisa
According to Leonardo's early biographer Giorgio Vasari, Leonardo had started to paint "Mona Lisa" in 1503, but "left it unfinished". However, a fully finished painting of a "certain Florentine lady" surfaced again in 1517, shortly before Leonardo's death and in his private possession. The latter painting almost certainly is the one that now hangs in the Louvre. Based on this contradiction, supporters of the authenticity of the "Isleworth Mona Lisa" claim it is the unfinished "Mona Lisa", made at least partially by Leonardo, and the Louvre "Mona Lisa" a later version of it, made by Leonardo for his own use.
Mona Lisa
Before the 1962–1963 tour, the painting was assessed for insurance at $100 million. The insurance was not bought. Instead, more was spent on security. Adjusted for inflation using the US Consumer Price Index, $100 million in 1962 is around $782 million in 2015 making it, in practice, by far the most valued painting in the world.