Prime cost sum. A prime cost sum (sometimes referred to as a PC sum) is an allowance usually calculated by the cost consultant for the supply of work or materials to be provided by a contractor or supplier that will be nominated by the client.rime cost sum. A prime cost sum (sometimes referred to as a PC sum) is an allowance usually calculated by the cost consultant for the supply of work or materials to be provided by a contractor or supplier that will be nominated by the client.