Please don't buy mining contracts without doing research on the economics of mining.
Here is a quick review of some of them. I assume these are legitimate, but I really have no idea. The returns do not include any fees. Most sellers of mining contracts quote numbers that assume that the difficulty will not rise. The fact is that the difficulty is rising dramatically.
Cloudhashing won't start mining until September. In the meantime, the difficulty is likely to double. A 2 GH/s contract costs about 1.5 BTC, and is likely to earn 1.6 to 3.0 BTC.
Bitcoin Frenzy also won't start mining until September. Their 1 GH/s contract costs about 0.5 BTC and is likely to earn 0.8 to 1.5 BTC.
MinerLease sells a 1 GH/s contract for about 1.3 BTC and it is likely to earn somewhere between 1.6 and 3.0 BTC.
The earnings listed here are more likely to be at the low end than the high end, and it is entirely possible that the contracts will lose money. It all depends on the difficulty.
An alternative to these mining contracts are the so-called "perpetual mining bonds" (they are not bonds), which are the same thing but they are traded on exchanges. At least with these, you are not locked in for a long time.
For a very comprehensive (and complex) discussion on determining the value of a mining contract, read this thread:
https://bitcointalk.org/index.php?topic=228327.0
