Small, regular withdrawals from the SiaPrime wallet usually renter allowance. When renter allowance is set up SiaPrime locks an amount that is taken from your wallet as contracts are created with hosts. If you don’t upload more than your allowance the unspent is returned at the end of the contract.
There is a general guide here (same setup, pricing is different on SiaPrime) https://siasetup.info/guides/renting_on_sia
Contracts are formed at the going rate and are locked until the agreed upon expiration. Our business customers are unaffected by underlying price changes while hosts accept the risk of coin price volatility. Storage nodes typically adjust pricing to match current exchange rates.