My logic for making profit in cex.io:
Assume you buy 1GHS today @ 0.0072BTC.
1GHS makes approx. 0.00005 BTC/day in today's difficulty (not counting maintenace fee)
So in order to stay profitable, you could either sell the 1 GHS today for >0.0072 BTC, or tomorrow for >0.00715BTC, or after 2 days for >0.0071BTC and so on.
Of course these figures will change after each difficulty adjustment, but logic stays the same.
The way I see it, the only thing that makes it a risky investment is the possibility of a website hack/shutdown etc. (think mtgox) since all the coins/ghs are online.
Ok so I'll try to take the fee into account.
From here
https://cex.io/maintenancefee is $0.26/month for 1 GHS
which makes 0.26/660 = 0,00039 BTC/month
which makes 0.000013 BTC/day
So for the current difficulty, each GHS makes a net of about 0.000035 BTC/day.
For the upcoming diffuculty, each GHS will make roughly 0.00003 BTC/day.