Given that all the costs involved in these are relatively fixed, its pretty easy to work out the potential profits. 
These units at this price will break even if the average difficulty change is around 1%. Anything more and they lose money.
Since that seems fairly unlikely, I think you need to significantly lower these prices.

It's pretty unrealistic to think that you could jump in and out of a poorly thought out $600,000 capital investment in a high risk field without any loses.