When did Athens begin to grow rich?

Answer 1

It began before the Persian invasion when a rich vein of silver was struck. It became entrenched after the end of the Persian War when Athens opportunistically turned the anti-Persian league of city-states which it led into a virtual empire of its own, diverting the annual war fund contributions by the cities to its own beautification and putting half its people on the public payroll to spread the loot. The funds were collected annually from the cities by its navy, by force where necessary.