I keep getting an error missing keyword after the When on line 3 I know I am doing something wrong in the case expression but can't figure out what.

select DISTINCT p.code, a.itemno, a.descrip, a.rev, a.class, a.std_cost, a.std_price, t.trans_in_out, t.trans_quan,
    (select case when V.MIN_CUST_PRICE is null then
    when V.MIN_CUST_NAT_PRICE is null then
    when V.MIN_SELL_PRICE is null then 0
    else V.MIN_SELL_PRICE
    else V.MIN_CUST_NAT_PRICE
    else V.MIN_CUST_PRICE
    end
    from v_sca_item_price_summary v where rownum = '1') as Value
    from arinvt a, translog t, eplant e, v_sca_item_price_summary v, prod_code p
    where v.arinvt_id = a.id
    and a.prod_code_id = p.id
    and t.eplant_id = e.id
    and t.trans_date between :start_date and :end_date