Manipulating SMALLMONEY data type

Hi,

I'm using the SMALLMONEY data type.  I have a function that uses a string value passed from VB.NET to add the string to the SMALLMONEY value and return the result as SMALLLMONEY.  The following function works:

CREATE FUNCTION dbo.AddMoney
      (
      @ORIGINALVALUE AS SMALLMONEY, --The original Value to be added to
      @ADDAMOUNT AS VARCHAR(10) --The value to be added
      )
RETURNS SMALLMONEY
AS
      BEGIN
      DECLARE @RETURNAMOUNT AS SMALLMONEY
      SET @RETURNAMOUNT = CAST(@ADDAMOUNT AS SMALLMONEY)
      SET @RETURNAMOUNT = @RETURNAMOUNT + @ORIGINALVALUE
      RETURN @RETURNAMOUNT
      END

My question is:  I need to calculate a percentage of a SMALLMONEY value and return the result as a SMALLLMONEY.  So I need to pass in the Price to amend, and the percentage to calculate ie (17.5% TAX).  What would be the best types to pass in to the function and how would I manage the conversions to get an accurate result?