Sales Order

Hi i created a sales order lets say january 10th with a price of $100 and the sales order did not ship untill two months later, our prices change ever month. is there a way i can configure my system for the sales order to pick up the new prices been proposed at a specific date at invoicing time? so in this case two months later the price would have gone up and i would like at time of invoicing the invoice to capture the new price and not the old price on the sales order. how can i do this?