What? No!
You do not seem to understand that whatever hashrate you may be using is ALWAYS competing with the TOTAL hashrate on the whole network.
You against all other hashrates, including others on cloudhashing.
Blocks are found at an overall constant rate.
Since the ammount you got from your contract at cloudhashing stays the same while the rest of the world is increasing at a high speed, the portion you get will become smaller. To earn the same you would need more hashrate to compensate for the fact that others have more hashrate too.
You should read more about the mining reward system in bitcoin.
In terms of amount per month, you're right. I'm talking about amount per block. Cloudhashing's pool discovers a whole block at once, and splits the value of that block among their customers. If the block reward stays the same, the pool hash rate stays the same, and the customer base stays the same, the amount of each payment stays the same. A 25 BTC block reward divided by 1000 contracts is always the same amount, aside from TX fees, which aren't very much.
Of course, the customer base won't stay the same, because people are always buying contracts. But since we know they haven't added any hardware, activating new contracts that can't be supported by the existing hardware is what would make the payments drop. Adding new hardware would also make the payments drop, but that would mean more frequent payments, depending on difficulty.