select
      a.idNumber              -- Presuming you want
    , b.idNumber              --    these too.
    , abs(a.Price - b.Price)  -- the price difference
from
    table a join table b on SOMETHING -- the primary key
where
        a.idNumber = b.idNumber
    and a.date = (select max(Date) from table t1
         where t1.idNumber = a.idNumber)
    and b.Date = (select max(Date) from table t2
         where t2.idNumber = b.idNumber)