Is it correct to assume that the price of cloud GH/S will vary proportionally with the difficulty change? For example, today on CEX.IO I can buy 1 GH/S for 0.08 BTC. From my understanding, a change in difficulty is imminent and will increase roughly by 20% (http://bitcoinwisdom.com/bitcoin/difficulty).

Will the price for GH/S decline by roughly 18.3% (1/1.2)?
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.