CREATE TABLE #T(Serial VARCHAR(4),PartID VARCHAR(9),Amt VARCHAR(10))
   INSERT INTO #T
  SELECT 'VC8', 'PC545', '4474.23' UNION ALL --note input modified
  SELECT 'VC00', '586', '225.00' UNION ALL 
  SELECT 'VC00', '516', '2550.00' UNION ALL 
  SELECT 'VC00', '421', '40.00' UNION ALL 
  SELECT 'VC9', '277', '19.23' UNION ALL 
  SELECT 'VC00', '815', '1620.00' UNION ALL 
  SELECT 'vc00', '206', '20.00' 
  
  SELECT Serial,SUM(CONVERT(DECIMAL(10,2),Amt)) FROM #T GROUP BY Serial
  Result:
Serial	(No column name)
VC00	4455.00
VC8	4474.23
VC9	19.23

;WITH SampleData (Serial, PartID, Amount) AS (
	SELECT 'VC8', 'PC545', '4,474.23' UNION ALL 
	SELECT 'VC00', '586', '225.00' UNION ALL  
	SELECT 'VC00', '516', '2,550.00' UNION ALL  
	SELECT 'VC00', '421', '40.00' UNION ALL  
	SELECT 'VC9', '277', '19.23' UNION ALL  
	SELECT 'VC00', '815', '1,620.00' UNION ALL  
	SELECT 'vc00', '206', '20.00' UNION ALL  
	SELECT 'VC1', '133', '348.76' UNION ALL  
	SELECT 'VC9', '436', '75.00' UNION ALL  
	SELECT 'VC9', '534', '273.76' UNION ALL  
	SELECT 'VC1', '133', '945.25' UNION ALL  
	SELECT 'VC9', '854', '945.25' UNION ALL  
	SELECT 'VC1', '826', '40.00' UNION ALL  
	SELECT 'VC8', '147', '40.00' UNION ALL  
	SELECT 'VC1', '754', '5,131.70' UNION ALL  
	SELECT 'VC4', 'PC545', '3,101.70' UNION ALL  
	SELECT 'VC9', '978', '2,030.00') 
SELECT Serial, PartID, Amount = CAST(REPLACE(Amount,',','') AS DECIMAL(8,2)) 
FROM SampleData
--WHERE Serial IN ('VC1','VC8')
ORDER BY Serial, PartID