I am keeping the transaction emails from coinbase when mining deposits hit my account and using that as the cost basis for each mining payment.    Not perfect but it will work.

My understanding is that since the process is a bit immature, the IRS is using some discretion with how people calculate their cost basis / fair market value, but they'll expect you to be consistent.  I.e. if you cant cherry pick exchange rates from different exchanges or times to help you setup a better cost basis. My advice would be to start a very thorough spreadsheet to track your income if you plan on making more than a grand per year.