CREATE TABLE QUOTE_TEST 
(
  ITEM_CLASS VARCHAR2(20 BYTE) 
, QUOTE_ID NUMBER 
, T_COST NUMBER 
, T_RESALE NUMBER 
) 
/

DML:

SET DEFINE OFF;

Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('A',1,1000,null);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('A',1,2000,null);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('A',1,3000,null);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('A',1,4000,null);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('A',1,5000,null);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('B',1,1000,1);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('B',1,2000,2);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('B',1,3000,3);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('B',1,4000,4);
Insert into QUOTE_TEST (ITEM_CLASS,QUOTE_ID,T_COST,T_RESALE) values ('B',1,5000,5);

commit;

Function:

    create or replace FUNCTION t_resale_test ( p_quote_id   quote_test.quote_id%TYPE ) RETURN NUMBER AS
        v_total_resale   NUMBER;
    BEGIN
        SELECT
            SUM(qt.T_COST * qt.T_RESALE)
        INTO
            v_total_resale
        FROM
            quote_test qt
        WHERE
                qt.quote_id = p_quote_id;

        RETURN v_total_resale;
    END t_resale_test;

Query:

select a.quote_id,a.item_class,sum(a.total_resale)
from
(select qt.quote_id,qt.ITEM_CLASS, t_resale_test(qt.quote_id)total_resale  from quote_test qt)a
where a.quote_id = 1
group by a.item_class,a.quote_id;

quote_id|item_class|total_resale
1       | A        | 0
1       | B        | 55000