It's a moving target. Back in the 1950's, a geophysicist by the name of Hans Pettersson tried to estimate how much meteorite material falls to Earth's surface each year. He took nickel samples from Hawaii's Mauna Loa by passing bits of dust through dust filters. Knowing that meteorites contain 2.5 percent nickel, he estimated that 14 million tons of meteoritic material settles upon Earth each year. Then we shoot stuff into space that leaves our gravitational pull causing us to lose some. Heck, who knows.

If we could just settle down and have a nice piece of PI.