Hi,
 is there somewhere a calculator or spreadsheet available so you can calculate the profit you made with the miner(s) you already have?

for example, in juni I purchased 20 USB erupters @ 0.5btc each, or at 75 euro each,
they have mined since then, what is my profit made with them.? (calculation with the known difficulties)

Is there a calculator that calculates historical profit and not only expected profit?