If the global field should always contain the same value, I'd use a stored calculation that always evaluates to that value instead of the auto-entered value.
Your calculation could also be made simpler:
Sum ( PRISM | Transactions | PV::Cost_Transaction ) + 0
with the "do not evaluate if all fields are empty" check box cleared
will also ensure that a 0 is returned by this field if there are no related records to sum.