You can do this using dynamic SQL, like so:

DECLARE @SQL nvarchar(4000)
DECLARE @BeginRange int
DECLARE @EndRange int

SET @BeginRange = 50
SET @EndRange = 75

SET @SQL = 'SELECT Element from ELEMENT_TABLE
WHERE Element IN
(SELECT TOP ' + CONVERT(varchar(3), @EndRange) + ' PERCENT Element
FROM ELEMENT_TABLE ORDER BY Value)
AND Element NOT IN
(SELECT TOP ' + CONVERT(varchar(3), @BeginRange) + ' PERCENT
Element FROM ELEMENT_TABLE ORDER BY Value) '

EXEC (@SQL)


HTH,
Stu