Hedera transaction & query fees are set in Fiat (USD) but paid in hbars. The rate of exchange we use between USD and HBAR is updated regularly based on (1) current market rates and (2) a defined minimum.  This structure is designed so that even if the fee paid in hbars for a given transaction may change, the fee in USD will remain stable. In this way, clients are protected from hbar volatility and given predictability in fee costs. When the price is low, the defined minimum rate protects clients against the fee in HBARs rising excessively (and potentially causing transactions to fail due to insufficient fee).