Is it correct to assume that the price of cloud GH/S will vary proportionally with the difficulty change?
The price of CEX.IO's GH/S asset is based on the total future expected payout of a Gh/s of mining hardware, factoring in management fees and the time value of money.  The current price already factors in future difficulty increases, as these are predictable and expected events.  The price should not move significantly when the difficulty adjusts, as the occurrence and magnitude of the adjustments are not a surprise to any informed participant in the market.