You run something where insiders set the price and outsiders can't check it.

A tariff from the Temple of Ba'al Saphon, set by thirty men in charge of fees and dated by the sitting sufet, priced every offering from an ox down to flour and oil, in silver shekels. Clans, guilds and drinking clubs paid by the same list. The poor paid the priests nothing, and a priest who charged off the list was fined.