GPS itself doesn't use much battery at all, nor do the calculations that use it. GPS-enabled apps use a lot of battery because they have to constantly wake up the device and enable the antenna, but that hardly matters for a navigation app, which is already awake and scanning constantly. Most of the battery use on Google Maps nav comes from keeping the screen on, and re-drawing the map it shows.

Source: I used to be the lead GPS engineer on this product.