Student or Learner
I am really worried about that we still speak English slowly. Before saying something in English, first we translate in mind what we want to say into English then we are able express something. Anyway! even now I am translating something that I want to say to my friend in English and it is something like this; how valuable do you think my phone is? Is it correct? TO make construct that I copied this construction; "who do you think you are?" which I often hear in films and songs. When talking, we have many problems to translate what we want to say in English.
Your sentence is fine. "How much do you think my phone is worth?" is another way to ask.