As a general rule I can confirm that using a cellular phone abroad for placing and receiving calls can get expensive. It wouldn't hurt to ask your wireless provider what the rates are. As for getting a Chinese cell just for your stay there - yes, this is a good option, but again before buying it you should inquire about rates charged.
Another option would be for you to use a calling card service. These usually charge much lower rates (we list such a service at the bottom of each country page - for example http://www.howtocallabroad.com/china/). You could either use this type of service from a local chinese landline or in connection with the throw away cell you intend to buy. Obviously the first option would be the least expensive, but people would not be able to call you.