The challenge with relying on ISNUMERIC() for this is that it can look at a lot of data that can't be summed and give it a pass. It isn't IS_ELIGIBLE_FOR_SUM() - it is merely returning true if the input could be converted to any of the numeric types. Commas are allowed because conversion to MONEY, for example, works successfully:
SELECT CONVERT(MONEY,'11,2');
So this also works:
SELECT SUM(CONVERT(MONEY,'11,2'));
Which probably isn't what you want. I think it would be safer to replace commas with periods before attempting to perform operations, or just exclude them and filter out rows this way:
DECLARE @t TABLE(n NVARCHAR(32));
INSERT @t VALUES(N'11,2'),(N'32'),(N'-32.4'),(N'32323');
SELECT SUM(CASE WHEN n NOT LIKE '%[^0-9.-]%'
THEN CONVERT(DECIMAL(18,2), n) END)
FROM @t
WHERE n NOT LIKE '%[^0-9.-]%';
The reason you have to perform a CASE expression in addition to the filter is that you can't control how the optimizer will process the statement - without CASE it might try to perform the conversion before the filter.
Of course, the smartest course of action would be to stop using the wrong data type in the first place. If you continue storing dirty data in ill-advised data types, you don't really get to complain about "optimal"...