Add conditional statement

ALTER FUNCTION dbo.udf_Pricing(@quantity int, @itemnumber varchar(50), @lowestprice bit)
RETURNS decimal (18,2)
AS
BEGIN
	DECLARE @Price decimal (18,2)
 
SELECT @Price =
 
-- One level
CASE 
	WHEN prc_or_disc_4 = 0 AND prc_or_disc_3 = 0 AND prc_or_disc_2 = 0
	THEN prc_or_disc_1
 
	-- Two levels
	WHEN prc_or_disc_3 = 0 AND prc_or_disc_2 != 0
		THEN CASE 
			WHEN @quantity < minimum_qty_2 THEN prc_or_disc_1
			WHEN @quantity >= minimum_qty_2 THEN prc_or_disc_2
		END	
	
	--Three levels
	WHEN prc_or_disc_3 != 0 AND prc_or_disc_4 = 0
		THEN CASE 
			WHEN @quantity < minimum_qty_3 AND @quantity < minimum_qty_2 THEN prc_or_disc_1
			WHEN @quantity <= minimum_qty_2 AND @quantity < minimum_qty_3 THEN prc_or_disc_2
			WHEN @quantity >= minimum_qty_3 THEN prc_or_disc_3
		END	
  
	--Four levels
	WHEN prc_or_disc_4 != 0 AND prc_or_disc_5 = 0
		THEN CASE 
			WHEN @quantity < minimum_qty_4 AND @quantity < minimum_qty_3  AND @quantity < minimum_qty_2 THEN prc_or_disc_1
			WHEN @quantity <= minimum_qty_2 AND @quantity < minimum_qty_3 THEN prc_or_disc_2
			WHEN @quantity >= minimum_qty_3 AND @quantity < minimum_qty_4 THEN prc_or_disc_3
			WHEN @quantity >= minimum_qty_4 THEN prc_or_disc_4
		END	
 
	--Five levels
	WHEN prc_or_disc_5 != 0
		THEN CASE 
			WHEN @quantity < minimum_qty_5 AND @quantity < minimum_qty_4 AND @quantity < minimum_qty_3  AND @quantity < minimum_qty_2 THEN prc_or_disc_1
			WHEN @quantity <= minimum_qty_2 AND @quantity < minimum_qty_3 THEN prc_or_disc_2
			WHEN @quantity >= minimum_qty_3 AND @quantity < minimum_qty_4 THEN prc_or_disc_3
			WHEN @quantity >= minimum_qty_4 AND @quantity < minimum_qty_5 THEN prc_or_disc_4
			WHEN @quantity >= minimum_qty_5 THEN prc_or_disc_5
		END	
END
 
FROM vw_Product_Pricing
WHERE Item_No = @itemnumber
RETURN @Price
END