How much (roughly) would it be for a bus from Shanghai, China, to Hanoi, Vietnam?

I have a flight from Canada to Bangkok, but a layover in Shanghai, China. I want to get off in Shanghai and spend a couple days there and catch a bus from there to Hanoi, Vietnam. However, I don't want to do this if it's going to be expensive. How much should it roughly cost for a couple nights in Shanghai and a bus ticket (I eat cheap, like rice with soy sauce for lunch and dinner)