What is the cheapest way to go from Shanghai SunIsland International Club to downtown?

Within walking distance, there is no suitable bus stop to catch for the downtown, so I suggest you take a taxi to cover around 3.5km to Shentailu, Xinguangcun bus stop first. Then take Zhujiajiao bus line 3 to Zhujiajiao Bus Terminal. There you can transfer to local Huzhu Special line to its terminal station, which happens to be in the downtown.