I have being to Sunday Market once. It's a bit far. The prices are pretty close to the on sale time in Central Market around 3pm on every Saturday afternoon. The owner of "Seven Brother" in the central market also sales at the Sunday Market (i.e. $2/ per box that one).
If you haven't being to Port Adelaide, you can go there for a short trip. Otherwise, you should just go to Central Market around 3 pm on each Saturday. However, Seven Brother only sales fruits & vegetables. If you want cheap seafood, you may just go on to jetty and do some crabbing & fishing. Ha~~~ ^_^