So, there is a point at which giving someone more money won't make them perform better and another point at which it makes them perform worse . . .

Some rough (US) data, keep in mind that these numbers are federal averages and should probably be adjusted by locality.

Average household size: 2.5

Poverty Level (P) for a household of 2.5: $16,440

Average income (A): $50,300

AMT is usually paid by those making over $150,000 (R=Rich)

So, the median income happens to be about 3x the poverty level, and one definition of rich is about 10x the poverty.

What is the salary above which performance no longer increases with reward? (M=Motivation Level)

What is the salary above which performance can be expected to deteriorate with increases in reward? (D=Deterioration Level)

Hypothesis anyone?