Again, it's an exchange. It's super fluid, and the price definitely varies. I've seen it as low as 0.040 and as high as 0.125 BTC per Gh/S. Hell, I've dumped all my Gh/S into BTC twice and have in no way lost money on the investment. Oh, and ROI? I have every 0.0001 BTC back + mining profits + trading profits. Yeah, lost a lot of money on that one

So long story short: Buy in and sit on CEX shares? Probably a loss. Trade, like a good member of a commodity exchange should? You'll head out on top. If you're smart.  

Cheers, fellows.