Convert hexadecimal values stored in string to integer equivalent

There have been a few occasions which  I have come across where its required to  convert hexadecimal value stored in string  value to integer. In such cases a simple cast  or convert wont work.Even converting to  intermediate varbinary will also return  wrong value. To handle such cases I've  written a UDF which I'm posting here
CREATE FUNCTION HexStrToInt  

(  @HexVal varchar (30)  )  

RETURNS int  AS  

BEGIN 

DECLARE @IntVal int  

;With Number_CTE(number)

AS

(SELECT 1

UNION ALL

SELECT number+1

FROM Number_CTE

WHERE number+1<=LEN(@HexVal)

)

SELECT  @HexVal=REPLACE(@HexVal,'0x','')  

select @IntVal= sum(case lower( substring( reverse(@HexVal), number , 1 ) )  

when '0' then 0  

when '1' then 1  

when '2' then 2  

when '3' then 3  

when '4' then 4  

when '5' then 5  

when '6' then 6  

when '7' then 7  

when '8' then 8  

when '9' then 9  

when 'a' then 10  

when 'b' then 11  

when 'c' then 12  

when 'd' then 13  

when 'e' then 14  

when 'f' then 15       

end *  power( cast(16 as bigint), number - 1 ) ) 

from Number_CTE

OPTION (MAXRECURSION 0)

RETURN @IntVal 

END   

and use it like   

select dbo.HexStrToInt('0x0000000000000028')