Voted Best Answer
Oct 31, 2017 - 07:09 AM
The grammatical rule says that when using "est-ce que" you follow the same order as in a statement, so it should be:
Combien est-ce que le billet de train coûte?
Combien est-ce que le billet d'avion coûte?
When asking the same question without using "est-ce que" though, the order would be reversed:
Combien coûte le billet de train?
Combien coûte le billet d'avion?
So people often mix up the two structures, as in: Combien est-ce que coûte le billet d'avion?
Thanks for pointing it out and sorry if we missed your question before, we'll fix this issue and publish an update soon.
*In the upgraded version of French, (in case you have access to it) we've actually re-structured the lessons on question structures to clarify the use of each one :)