"What kind of balls are not expensive?"
Translation:어떤 공이 안 비쌉니까?
Use English to help answer your question! A possible answer to the question would be "Red balls are expensive." So, what kind of balls are expensive? Red balls. 어떤 is practically acting as an adjective, hence why it goes before the noun like you've seen throughout the rest of the course! Hope this helps!
If the question word is acting as an adverb, it goes right behind the verb at the end. However in cases like this, where the question word is acting on a noun, it goes before said noun. Hope that made sense. I had the same problem for a bit had to have a korean friend explain it to me -.-