Minimum deposit amount is only 0.0005 BTC. Minimum withdrawal amount is 0.001 BTC.
The speed of mining directly depends on the number of contracts purchased and their claimed capacity (GH/s). The more expensive the contract, the less the cost of 1 GH/s. So, when you buy a more expensive contract, you save on the cost and in the end get more profit. For example, it would be more profitable to purchase one contract with a claimed capacity of 150 GH/s than buying 3 contracts with the capacity of 50 GH/s.
