Try this:
DECLARE @tmp TABLE (Col1 INT IDENTITY(1,1), Col2 INT)
INSERT INTO @tmp (Col2)
SELECT 50 AS Col2
UNION ALL SELECT 100
UNION ALL SELECT 50
UNION ALL SELECT 100
SELECT CONVERT(VARCHAR(30), Col1) As Col1, Col2
FROM @tmp
UNION ALL
SELECT 'Average', AVG(Col2)
FROM @tmp
Result:
Col1 Col2
1 50
2 100
3 50
4 100
Average 75