I don't understand why people say you loose money at cex.io, If you purchase $500 in GHash and let them mine for you, any and all btc they mine is profit! If it looks like the bottom is going to fall out of the market, Just sell the GHash and get your $500 back, You may get a little less or a little more depending on the Price of the GHash when you sell.. You are still making $$$$ while the GHash you purchased is mining for you!!! This is the way I see it anyway! But everyone has their own ideas about things!
Have you ever heard of a little something called the difficulty increase? Haha it's not as simple as spending some money on gh/s, mining for a while and then selling the gh/s for the same price as you bought them for. The difficulty increases roughly every 2 weeks and when that happens your gh/s become less valuable. Also, the price for gh/s on cex.io is usually pretty high to begin with, not to mention the fees that they throw at you... Mining just simply isn't profitable at the current price of btc. However, the best way to wrap your head around it is to try it. You learn really quickly that it isn't the easy money that it initially sounds like it's going to be.