The calculations are correct, but this is based on the current difficulty. Difficulty on bitcoin increases at an astonishing rate, i think someone mentioned 116% per month if i am not mistaken, but for example on the 22nd of December it was around 900,000,000 while now its 1,789,546,951. it has almost doubled in less than a month.

There are some better calculators out there that allow to input a monthly diff increase percentage, that calc would bring you some more realistic values.
e.g. http://bitcoinwisdom.com/bitcoin/calculator

Once you input at least 100% into the difficulty increment you will see the realistic values.

At the current moment in my opinion its too late to jump the bitcoin wagon with asics, the asic you buy now will be useless in 3 months (well for BTC at least) so if you do not manage to make a return, then you will be losing out instead of profiting.
You could attempt to mine other SHA256 coins, but what i found was that when bitcoin diff jumps, people jump to other SHA 256 coins and once the diff is increased there they just jump back to btc mining.

If i was you with money to spare, i would seriously consider mining Scrypt coins, there is a much bigger choice (well most are junk anyway) but you will have a more realistic return to every dollar spent mining dogecoin for example (which has tripled in its price in the last 72 hours)