I don't think the difficulty has anything to do with what you buy or sell your ghash for on cex.io, I bought my ghash more than a month ago @ 0.00646300 btc and now I can sell them for 0.00765999 btc ,  There is no difficulty in that! There is difficulty in the mining ghash do, and that goes into figuring how much my payout is for the ghash I have working. Not the ghash themselves!

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!