It depends on the flight length. The price difference (at same fare) between goLight and Flex is anywhere between 15 (?) and 45 (?) euro. So I would expect that the costs would be:
- Price difference between GoLight and Flex (15 to 45 euro)
- Fare difference (if any, may be possible to stay within same fare even if already sold out?)
- Call centre fee (20 Euro)