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The value of 3GH/s (0% downtime, 0% stales, 0% pool fees) is ~ 58 BTC per 30 days.
http://www.alloscomp.com/bitcoin/calculator.phpSomeone looking to purchase a contract would be paying less than that how much less depends on how desperate you are willing to sell and how much they trust you.
In essence a mining contract is simply a loan paid back on a share basis. The "interest" determines how much less that expected value the contact sells for.