After you take all these in consideration you can make a real calculation:
Oh, God... Why all of you believe that exchange rate for 1 BTC will always be 400+$?... What makes you think so?..
And why do people think the exchange rate has ANYTHING to do with hardware purchases.
If it mines less btc than it costs you've lost regardless of what happens to the rate.
Spend
BTC3.5 and get back
BTC2 and you are
BTC-1.5
If you are lucky and BTC doubles to $800 and you cash out yes you will have made a small profit,
but if you hadn't bought the thing in the first place you'd have made a much bigger one.you sell your bought/held BTC at the exact right time.