declare @MyTable table (    -- Stuff you should be supplying
    code  int not null,
    month char(6) not null,
    value int not null
    )

insert into @MyTable (code, month, value)
values
    (1234, '201201', 75),
    (1234, '201202', 95),
    (1234, '201203', 80)

;with Ordered              -- Possible solution
as (
select code, month, value, row_number() over (partition by code order by month) rn
from @MyTable
)
select
    a.code, a.month, a.value - b.value value
from
    Ordered a
inner join
    Ordered b
        on  a.code = b.code
        and a.rn = b.rn + 1

SELECT t.code, t.month, t.value - t1.value AS value
FROM table t
CROSS APPLY (SELECT TOP 1 value
             FROM table
             WHERE month < t.month 
             ORDER BY month DESC)t1