Is 超度 the only thing the monk can help in this case?
Asking for those who need this sort of service, chanting - how much
must people pay for this?
If like you said, if the spirit wants revenge, then what can
buddhists do to send the spirit away?
No, what I know the monks have other ways besides 超度.If the
spirit is too fierce, they will "catch" the spirit instead of
the soft approach. Usually the monk will just take "ang pao", any
amount will do. It is better to send the spirit away as soon as
possible, any delay is no good for the person who is possessed.