can you buy musical instruments cheaply in shangai or beijing?

To get cheap such items in Beijing, I suggest you go and have a try at Anding Gate, Inner Street (Andingmen Neidajie). There you can find the musical instruments shops.

In SH city, I recommend East Jinling Road to your, which is considered the local Musical Instruments Street. You can find many such shops there and hope you can find the cheapest one. And bargaining is necessary~