this is all great and fine with everyones examples of 1 BTC...but what about the other 99% of us who mine like .1/.2/.3 btc/day for example.
We're now supposed to keep track on every fraction of BTC we receive as well as the current price we receive that at? that's almost impossible.
Look at your receiving address(es) that you use for mining on blockchain.info. It will show all of the time and amounts of received btc. I'm pretty sure that the blockchain.info api has a function to show what it was worth at the time of the transaction, since my android blockchain wallet will tell me the "now" and "then" values in USD on each transaction.
Not impossible if you have a computer connected to the internet and your receiving address(es).