Theoretically, negative interest rates' lower bound depends partly on the cost of holding cash in the form of physical currency. When people hold cash out of aversion to negative interest rates, they risk losses due to theft and the like. The cost of avoiding this risk could be a key determinant of negative interest rates' lower bound, but it is hard to directly quantify. As a proxy for the cost of holding physical currency, we estimated the cost of storing gold based on gold futures prices. This cost has averaged an annualized 2.4% over the past 20 years, though it has varied widely over this timeframe.