A typical low-poly model would run you about 500-1000 polygons.
Low-poly modeling is essentially playing connect-the-dots to represent a convincing silhouette of your wanted object. And then being creatively stingy with the number of said dots.
Notice the Medic's bonesaw. You can use alpha maps in your textures to save a fair number of vertices that would've been otherwise wasted representing the curved edges of the object which in the end, probably wouldn't of had looked as good. That's an example of being creatively stingy while making low-poly models.