I use Google Spreadsheet to track my income and outcome.

I take care to note every expense, with time, the coin used to pay and the value of that coin compared to EUR, USD and BTC.

So, the price is fixed at that time, and, I can see exactly the amount I have received or spent in a month.

My spreadsheet template is almost available, but right now in spanish only (I need to translate to english).

Let me know if you want it.