Holy matrimony is both a civil and an ecclesial entity – it has both to do with the state and with the church. A marriage service thus has to take both civil and canon law into consideration and you need to check with each of the Porvoo churches individually, as with your Embassy, if you wish to marry in any of the Porvoo countries.
What does it cost?
The Porvoo Agreement is an ecumenical, not an economical agreement. This means that rules regarding fees for different ecclesial services differ from Church to Church.