40 feet = 12.192 metres.
Volume of cube = 1812.278 m^3
Density of gold = 19300kg m^-3
Total mass of gold in cube = 34976968.91 kg
1kg = 32.1507474 oz Troy
Price of gold (BBC) = $875/oz Troy
Total price of gold in cube = $9.84^11 or 984 billion dollars.

Or approximately the cost of the Iraq war by the time they pull out.

Howstuffworks estimates that "the world produces a cube of gold that is about 4.3 meters (about 14 feet) on each side every year." At that rate, Asimov's 40 metre cube would represent about 23 years' production, so, it's probably an underestimate.