I ask myself the same question...

If you got it online today it would take you 126.3 days to mine your BTC back with the assumptions

- You have free power
- There is an average of a 5% difficulty rise across those days

If the difficulty goes up more than that.. and you have to pay for power

You're easily looking at 150-200 days to possibly break even.. depending on your power costs (you may not ever break even)

Save yourself the hassle and just buy 3.9 coins....