case
when ProdGrpCode in ('500','510','5201','580','630','460','470','480','490') then '82' 
....

declare @myVar as varchar(50)
set @myVar = '500, 510, 5201, 580, 630, 460, 470, 480, 490'

case
when ProdGrpCode in (@myVar) then '82' 

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE FUNCTION [dbo].[DelimitedStringToTable](@String varchar(8000), @Delimiter char(1)) 
       
returns @temptable TABLE (items varchar(8000))        
as        
begin        
    declare @idx int        
    declare @slice varchar(8000)        
       
    select @idx = 1        
        if len(@String)<1 or @String is null  return        
       
    while @idx!= 0        
    begin        
        set @idx = charindex(@Delimiter,@String)        
        if @idx!=0        
            set @slice = left(@String,@idx - 1)        
        else        
            set @slice = @String        
          
        if(len(@slice)>0)   
            insert into @temptable(Items) values(@slice)        
  
        set @String = right(@String,len(@String) - @idx)        
        if len(@String) = 0 break        
    end    
return        
end

GO

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

case
when ', ' + @myVar + ', ' like '%, ' + ProdGrpCode + ', %'  then '82' 

CREATE TABLE #T (ProdGrpCode varchar(10), ResultCode VARCHAR(10))
INSERT INTO #T
SELECT '500', '82' UNION SELECT '510', '82' UNION SELECT '5201', '82' UNION 
SELECT '580', '82' UNION SELECT '630', '82' UNION SELECT '460', '82' UNION 
SELECT '470', '82' UNION SELECT '480', '82' UNION SELECT '490', '82'

case
when ProdGrpCode in ('500','510','5201','580','630','460','470','480','490') then '82' 
....

/* Table valued function for parsing a delimited array into a table */

CREATE FUNCTION [dbo].[tvfParseDelimitedString] 
    (
     @S NVARCHAR(MAX)    -- Delimited input string
    ,@Split CHAR(1)      -- Delimiter used for the input string
    )
RETURNS @Table TABLE 
    (
     [ID] INT NOT NULL IDENTITY(1,1)
    ,[Value] NVARCHAR(MAX) NULL
    ,PRIMARY KEY ([ID])
    ,UNIQUE ([ID])
    )
BEGIN

    DECLARE @X XML

    SET @X = CONVERT(XML,'<root><s>' + REPLACE(@S,@Split,'</s><s>')+'</s></root>')
    
    INSERT INTO @Table
    SELECT LTRIM(T.c.value('.','NVARCHAR(MAX)')) AS [Value]
    FROM @X.nodes('/root/s') T (c)

    RETURN 
    
END


/* Create a set of test data */

IF OBJECT_ID('tempdb..#TempTable') IS NOT NULL
DROP TABLE #TempTable

CREATE TABLE #TempTable (ProdGrpCode INT)
INSERT INTO #TempTable
SELECT '500' UNION SELECT '510' UNION SELECT '5201' UNION 
SELECT '580' UNION SELECT '630' UNION SELECT '460' UNION 
SELECT '470' UNION SELECT '480' UNION SELECT '490' UNION
SELECT '777' UNION SELECT '888' UNION SELECT '999'


/* The final query that takes the delimited input */
/* and assigns the proper code                    */

DECLARE @myVar AS NVARCHAR(50)
SET @myVar = N'500, 510, 5201, 580, 630, 460, 470, 480, 490'
    
SELECT 
    ProdGrpCode
    ,(CASE
        WHEN ProdGrpCode = Value THEN 82
        ELSE 99
     END) AS Code
FROM 
    #TempTable AS t
LEFT OUTER JOIN
    (SELECT Value FROM dbo.tvfParseDelimitedString(@myVar,',')) AS v
    ON t.ProdGrpCode = v.Value