You need to use .
as decimal separator for 0.25
:
(SALES_PRICE * 0.8 * (1-u.discount) * 0.25) as VAT
To get only 2 decimal places, you can do CONVERT(DECIMAL(30,2), yourvalue)
.
To get the total, just multiply by 1.25 (SALES_PRICE * 0.8 * (1-u.discount) * 1.25)