According to http://www.bitcoinx.com/profit/, as a Batch 2 customer, I expect to nearly pay for my original Neptune unit by the end of year including power costs. My profit will be found in 5 months of mining with the second unit, and in selling the used units at a greatly reduced rate when I'm done with them.

Of course, any predictions beyond a couple months are difficult to make with accuracy. And as with most investments such as stocks etc., "past performance is not indicative of future earnings". No way to be sure of anything.