You have to keep track of everything
If you mine them the price you got the BTC is 0$ + costs; if you sell them on an exchange, the selling price is what you get and if you buy something with the BTC it is the market price of that item
When you sell you use the btc you got the cheapest or the price of the oldest coins first or if you want to gamble with the irs you use a price average
More infos here from a us tax lawyer :
http://www.reddit.com/r/Bitcoin/comments/1uccfz/i_am_a_tax_attorney_here_are_my_answers_to_the/Yeah but if you mine other coins than BTC it adds a whole extra layer of complexity, since altcoin prices are usually denominated in terms of how much BTC they are worth, NOT how much USD, adding a whole step to the process as I described.