The number we will buy depends on how expensive
　　　 is their product.

　　The number we will buy depends on how expensive
　　　 their product is.

　　　　我們會買多少得看他們的產品有多貴。

Depend on 後需接受詞，其後若為一問句，需改為間接問句
（即名詞子句），換言之，主詞 their product 與動詞 is 需倒裝。