Stuff like cex.io is very unpredicatable and it is gambling basically. Just wanted to tell you all.
Why do u think cex.io or any cloud mining service is gambling ? Profit can be calculated by difficulty estimation. There are many tools to do that as well.
Difficulty cannot be estimated in the long run. You can only try to estimate the next difficulty increase but it may or may not be accurate. In a few months after buying the GHS, the difficulty may have rise as more ASICs are being shipped or big farms coming online. If that happens, you would profit much less from your overpriced GHS from cex.io and you can't sell it for much value as the price would be lower.