Bonsoir Rene, En parlant français formel, la forme correcte est : «Combien d'euros est-ce que ça coûte?» ou «Pourquoi est-ce que vous voudriez faire ça». Mais, d'habitude, au cours d’une discussion la forme changerait un peu parce que c'est plus informel. La forme la plus formelle est l'utilisation de l'inversion du sujet-verbe.

"Où est-ce que tu vas" sounds a bit stilted when used in a normal conversational context. That's because you'd much rather use "Tu vas où?" our even "Où vas-tu?". Shorter and more to the point. Although "où est-ce que tu vas" is grammatically perfectly alright. Sometimes the inverted form, although considered in general to be more formal, is the expected form. As in, "Comment vas-tu?" wich is used this way even on informal occasions. It is probably best to be well versed in all 3 forms of questions and then start reading and participating in spoken French. You'll get a feeling for when which form is more appropriate and "expected" by the native French speaker. -- Chris (not a native speaker).