How to make the trip of Beijing - Shanghai - Beijing on a budget?

We have a RT ticket Manila-Beijing-Manila for 2 adults and 2 kids. We would have wanted to make Beijing our point of entry and exit at Shanghai but the promo available is only for Manila-Beijing-Manila route. We'll be arriving Beijing on April 4, 2011 at 0035 am and depart on April 11 at 0100am. Is there any way for us to be able to make a round trip to Shanghai on a budget? Since we'll be in Beijing anyway, it would be an opportunity to see Shanghai as well. I really hope somebody can give me tips on how to make that side trip on a budget coz I really don't want to miss that opportunity if possible... Thanks very much.