SQL GROUP BY CASE statement with aggregate function

I have a column that looks something like this:

CASE
    WHEN col1 > col2 THEN SUM(col3*col4)
    ELSE 0
END AS some_product

And I would like to put it in my GROUP BY clause, but this seems to cause problems because there is an aggregate function in column. Is there a way to GROUP BY a column alias such as some_product in this case, or do I need to put this in a subquery and group on that?