sum 1500$ * (1.246^i / 1.372^i), i=1 to n

n = number of months (use wolframalpha to play with the formula)

Example #1: 12 months from now you will be making 472$/month for a total of 10,164$. Not bad for a 3800$ miner...

Example #2: at 226$ per month in electricity bills, the rig will become obsolete in 17 months (September 2015).