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!