If you just want nav, and have a laptop, get a gps receiver with trip software for like $80.
Click here!
Not as cool, obviously not integrated, but does the job - with audio turn by turn directions, on the fly trip planning and the like... I do this for work as I'm on the road a lot and refuse to spend 2 grand on a headunit...
Even if you don't have a laptop, you could buy a low end cheap one, plus a receiver and still be half the price of most units on the market ;p