Ok. I asked this in another thread already, but HERE, I particularly do not understand where a lot of you are coming from.
First off, no matter what you WILL achieve ROI, so I have to assume you mean POSITIVE ROI. But even given that niggle, there is virtually no way these machines will not reach break even in a fairly short time, after which your only ongoing expense is electricity, which is not that great of an expense on these devices.
So, those of you crying "no ROI" over and over again, WHAT FREAKIN" TIME FRAME ARE YOU TRYING TO BEAT?

Any frickin' timeframe. Please see
here.
I mean really. An ivestment that is likely to break even in less than a year??? That's insanely positive by most any business metric you care to apply! ...
If you don't break even in a year, you never will. Never. Unless conventional difficulty predictions are just a bunch of fud, spread by evol gobment infiltrators.