Thanks 212man. Looking at the article, in Heliflyer's case,
Article 130, para 8 (ii) (bb); The proportion which such contribution [I'll assume this means the passenger's contribution] bears to the total direct costs of the flight shall not exceed the proportion which the number of persons carried on the flight (excluding the pilot)[3] bears to the number of persons carried on the flight (including the pilot) [4].
Therefore, the pax can't pay more than 3/4 of the cost of the flight.