Various places in minergain show different things for the US dollar piece.  For example on the rewards area, we show the dollar price as if you sold it immediately and the current price if you held it and sold all of it right now (giving a gain or loss).  On the home page I am pretty sure I am just showing the coin value without a fiat value.  On the coin price list, it gives the fiat price for the coin at this exact moment in the currency you specify (about 2 dozen currency options).  I am probably going to add in some way to import / enter exchange buys / sells along with scanning the blockchain for mining rewards to try to figure short / long term gains.  I tried a couple other web options out there but I am not thrilled about the hundred dollars per year just to track that.  And since I have coin prices for the past couple years, it would not be that hard to do (other than the user interface).  If anyone else is interested in that sort of capability, let me know with a personal message or click the About Us / Contact Us link.