Comparing an NTEXT field against a fixed value without type errors

The ntext type has been deprecated, with nvarchar(max) as its replacement. Changing the table's column type would be the preferable fix, if that's possible. Comparing against a varchar literal then causes no difficulty.

Failing that, you'll need a cast of the value ahead of the comparison:

cast([longtext3] as nvarchar(max))
  <> '2,048.6'

Another thing worth trying is an nvarchar literal, which addresses certain related type issues:

cast([longtext3] as nvarchar(max))
  <> N'2,048.6'

When avoiding a cast is preferable, LIKE or PATINDEX can work in certain situations, as this MSDN discussion demonstrates: http://social.msdn.microsoft.com/Forums/en-US/transactsql/thread/6bd4c661-ea0b-435f-af78-097e61549d41

Without wildcard characters, the LIKE expression would approximately match an equality check here.

For this particular example and its comparison, you would write:

[longtext3] NOT LIKE
  '2,048.6'