Try this:

select t1.pid, t1.sales, t1.month1,
isnull((select t2.sales from table1 t2 where t2.pid = t1.pid - 1) - t1.sales, 0) as differe
from table1 t1

Depends on your database and version thereof.
If it's new enough you can use the analytical function Lag

Select  Pid,
        Sales,
        Month1,
        LAG(sales, 1, 0) OVER (PARTITION BY Pid ORDER BY Month1) - sales as difference
FROM    MyTable

For other solutions you need to state your Database and model as it differs a bit between them

<update>
You can also use Row_Number:

With ordered as (
    Select  Pid,
            Sales,
            Month1,
            Row_Number() OVER (PARTITION BY Pid ORDER BY Month1) rn
    FROM    MyTable
    )
Select  O1.Pid,
        O1.Sales,
        O1.Month1,
        O2.Sales - O1.sales as Difference
FROM    Ordered O1 left outer Join Ordered O2 on O1.rn = O2.rn - 1

</update>