MS SQL : Use Computed value to Compute other values
It's simple, computed value can be used by applying is value with cross apply

like this
select cpk.coupon, p.Price, p.Price-cpk.coupon as FinalPrice -- etc

from Product -- etc
CROSS APPLY (
   select coupon = CASE 
  WHEN c.CouponPercent = 0 THEN p.Price - (c.CouponPercent/100)
  ELSE c.CouponValue
END as Coupon ) as cpk

where --etc