Math fact:

The rate of total stacking is quadratic (x^2), even though the stack amount increase is linear (x).

Example:

If the last person stacks $420, then the approximate total stacked is on the order of $420 x $420, which is around $100,000.

Result:

The actual equation for total stacked is x(x-1)/2, where x is how much the last person stacked. So for the example above, the total stacked amount is 420*419/2= $87,990 which is close to the approximate answer.