Agriculture was the main source of income in the Persian Empire. Most of the produce was sent to the government for tax purposes.
Persian taxes became heavier and more oppressive. Thus, leading into economic depression and revolts, which in turn led to heavier taxes, and so on. Rulers began hoarding silver and gold, rather than recirculating it, creating economic destruction because there isn't enough for business. The Persian kings got wealthier, feeding back into the revolts, and so on.