I'm traveling to Beijing in March and I want to take a side trip over a weekend to Shanghai.

I see people recommend buying the flight tickets locally in Beijing; where should I do it? At the airport upon arival, or somewhere in the city?

Also, I see some people say that tickets from Shanghai - Beijing are expensive; will we be able to get cheap roundtrip tickets out of Beijing, or are they going to be cheap one-ways to Shanghai and expensive returns to Beijing?

Thanks!