I think it drains the battery a lot because the GPS position and map drawing occurs very often so that the position updates fast enough in town at corners and roundabouts etc. so that the "Take the next exit" prompt is reliable.
Even if the GPS is not updated too often (interpolation works pretty well in tunnels), the screen updates would definitely drain the battery, just like video games would.
For the record, mine doesn't drain while it is on (wired) charge in the car using nav.
