How do I determine the cost for capital gains purposes of I-shares of silver?

I have downloaded capital gains amounts from my brokerage. One of the investments included is in I-shares of silver. Turbo-tax requires that I fill in their cost. Can someone tell me please how I would ascertain these values. TT highlights their omission as an error on multiple forms. Is it possible that the IRS does not require this information as it may not be relevant to this type of investment?