Do you really mean a "hammock" task which is created by pasting links to
start and finish dates or do you mean a summary task. In the first case, the
hammock task has no mechanism to sum tasks. In the second case, the fixed
cost field lets you assign a fixed cost to a task. Summary tasks can have
their own fixed costs so the value is NOT the sum of the subtasks. However,
the COST field for a summary task WILL show the sum of all the costs
including fixed costs.
It is a bit confusing, but just remember that the fixed cost field is a
place to assign the cost rather than to track it.
-Jack Dahlgren
http://zo-d.com/blog