If you just follow the surface with retopo tools your asset is to flat. You have to model in bigger more exaggerated shapes by hand. For example make the nose and ears bigger.
400 is pretty low for a full body, so you would need to make the head pretty much a blob. But if that's the budget for the head alone, you could add more detail in eyes, mouth, ears. Some lowpoly examples here http://wiki.polycount.com/wiki/FaceTopology http://wiki.polycount.com/wiki/BaseMesh