A fiduciary is legally obligated to act solely in your best interests at all times. You can't depend on sales commissions and other third-party payments that subsidize the cost of financial advice. The exact cost varies by situation and region. For example, in Pinal County, Arizona, the suggested maximum fee schedule for fiduciaries includes a $300 flat annual fee for estates of $100,000 or less and a management fee cap of 3%.