WHEN RunningTotal >= ReceivedQuantity THEN increment to next Purchase Price.

CREATE TABLE dbo.TEMP_PurchaseOrder
(
   POID               NCHAR(7)      NOT NULL
  ,ItemID            NCHAR(1)     NOT NULL
  ,RecievedDate      INT           NOT NULL
  ,Received_quantity DECIMAL(17,6) NOT NULL
  ,Purchase_price    DECIMAL(17,6) NOT NULL
);

INSERT INTO TEMP_PurchaseOrder
VALUES
     (1000100,1,'20120401',10.000000,5.100000)
    ,(1000104,1,'20120410',10.000000,5.100000)
    ,(1000105,1,'20120420',10.000000,5.050000)
    ,(1000101,2,'20120401',10.000000,15.000000)
    ,(1000102,2,'20120401',10.000000,15.000000)
    ,(1000103,2,'20120409',10.000000,14.500000);

CREATE TABLE dbo.TEMP_Transactions
(
  OrderID      NCHAR(7)      PRIMARY KEY
  ,ItemID      NCHAR(15)     NOT NULL
  ,TransDate   INT           NOT NULL
  ,POID        NCHAR(7)      NOT NULL
  ,TransQuant  DECIMAL(15,6) NOT NULL
);

INSERT INTO TEMP_Transactions
VALUES
     (1000100,1,'20120401',100,10.000000)
    ,(1000002,1,'20120401',0,-5.000000)
    ,(1000003,1,'20120401',0,-2.000000)
    ,(1000004,1,'20120407',0,-3.000000)
    ,(1000104,1,'20120410',104,10.000000)
    ,(1000006,1,'20120412',0,-1.000000)
    ,(1000007,1,'20120415',0,-8.000000)
    ,(1000105,1,'20120420',105,10.000000)
    ,(1000101,2,'20120401',101,10.000000)
    ,(1000102,2,'20120401',102,10.000000)
    ,(1000011,2,'20120402',0,-5.000000)
    ,(1000012,2,'20120405',0,-2.000000)
    ,(1000013,2,'20120405',0,-4.000000)
    ,(1000014,2,'20120406',0,-5.000000)
    ,(1000015,2,'20120408',0,-1.000000)
    ,(1000103,2,'20120409',103,10.000000);

Begin
    ;With trans as
    (
      Select distinct
           TransDate
          ,ItemID
          --,POID
          ,RunningSold = abs(SUM(sum(case when POID = '0' then cast(TransQuant as Int) else 0 end)) OVER (PARTITION BY ItemID Order by TransDate ROWS UNBOUNDED PRECEDING))
          ,RunningRecieved = SUM(sum(case when POID > '0' then cast(TransQuant as Int) else 0 end)) OVER (PARTITION BY ItemID Order by TransDate ROWS UNBOUNDED PRECEDING)
        From dbo.TEMP_Transactions
        Group by TransDate,ItemID--,POID
      )
        SELECT distinct  t.TransDate, t.ItemID, (t.RunningRecieved - t.RunningSold) as RunningTotal
        FROM trans t
        Where t.ItemID = '1'
        ORDER BY t.TransDate, t.ItemID;
END

DROP TABLE dbo.TEMP_PurchaseOrder;
DROP TABLE dbo.TEMP_Transactions;