Hi Dipak,
I think while creation of Contract the SAP have the funcinality of defining the condition type in item wise with there respective validity date
i.e Material :- Gold
20.09.2013----23.09.2013------- 31
24.09.2013---30.09.2013-------38
You have to select the line item in Contract and click the Item condition tab where you can define the price of material for perticular period . also you can maintain the future price in Material master in future price