Ok, for the sake of theoretical argument, lets hold BTC/USD at 15. Say the difficulty stays where it is right now. Once we get a number, we can adjust it for these variables.
We can't because difficulty changes every 14 days at 6 blocks/hour, or about every 5-9 days at 7-12 blocks per hour as it has recently.
Difficulty wont stay at 1,3 million just as it didn't stay at 877k.
It will rise to about 3-5 million by the end of next month which means any calculations made at current difficulty are completely useless.
You can't calculate future earnings based on 1,3M difficulty when it's only going to last about a week before going up again (and likely double within 20-30 days).
If BTC price really does stay at $15/btc then a miner buying a full rig today will likely never pay it off. He's missed the train by a long shot.
His only hope is holding onto what little he can get out of his rig & wait for price appreciation.