Learn German: Warum ist es denn so überteuert? - Why is it so overpriced then?

As a modal particle in questions, denn softens the tone and shows genuine interest or mild surprise. Without it, Warum ist es so überteuert? could sound a bit aggressive or like a strict interrogation. Adding denn makes it sound more conversational, similar to adding "tell me" or "then" in English.

Wait, doesn't denn mean "because"?

Yes, when used to connect two clauses, denn means "because" (for example: Ich gehe, denn ich bin müde). But in this sentence, it isn't connecting two ideas. When denn is dropped into the middle of a question like this, it acts as a "flavor word" (a modal particle) to adjust the tone, not as a conjunction.

Why is denn placed right after es?

Modal particles usually sit in the middle of a sentence. They naturally come after the conjugated verb (ist) and any pronouns like the subject (es). They almost always come before the main descriptive part of the sentence, which here is so überteuert.

How is the word überteuert formed?

It is built around the basic adjective teuer (expensive). The prefix über- adds the meaning of "over" or "excessive." The word is actually the past participle of the verb überteuern (to overcharge), which is why it ends in a -t, just like the English equivalent ends in "-d" (overpriced).