If you were to collect it all in and send it all off to Money4Gold they would offer you £21.33 for it.
You can always make a stab at these things by breaking the problem down. World population is 6.8 billion. Ignoring kids under 10 the population is around 5.5 billion. Of these 1 billion are in developed countries and 4.5 billion in less developed. Taking an average of £500 per person (aged over 10) in developed and £100 in less developed countries (my guesses- could be miles out) that's a total of £500b + £450b= £950b
So we are looking at around a trillion dollars.
But then I'd probably double that because of all the unsold stock, government reserves of gold and some ultra high value collections.
So maybe approaching 2 trillion.
Why do you ask