For example, if i expect 1 bitcoin 10 years from now, I sell a future contract promising to sell 1 bitcoin at $45,000. If the price falls 5,000 over 10 years, the value of my contract is now worth $5,000 plus the 40,000 value of the coin. If I am on the opposite side and have to pay in bitcoin, I can buy a futures contract for 1 bitcoin at 45,000. If the price of bitcoin rises I am offset by the value of the futures cotnract and if the price falls the negative value of the futures contract locks my price at ~45,000.
The cost here is the premium of the futures contracts, which of course could make it more expensive to operate a crypto mortgage in the long run.