they inquired ''how much does it cost?'' -> reported speech

Answer

`->` They inquired how much it cost.

`-` Câu tường thuật NV (Wh-Question) `:`

`->` S + asked O / wanted to know / wondered + Wh-Ques + S + V (lùi thì)

`-` Lùi thì: HTĐ `->` QKĐ `( + )` S + V2 / V-ed - Quá khứ của cost là cost