Welcome to NICO!
If it is just the computer controlling the system, it may not be too bad a price - although labor tend to be the most of the cost. However, if the entire NAV system is non-functional, this is apparently an expensive repair if you are not in warranty.
Yes, I would strongly recommend extended warranties on modern cars with lots of electronics ... the repair costs tend to be pretty darn high otherwise. This is not unique to Infiniti, by the way.
As one alternative, you can occasionally find used electronic components at junk-yard places - not always, but sometimes. You may need to call around a lot of places - not always local to you.
Finally, consider giving Joe Tsensky at
http://www.everythinginfiniti.com a call if all else fails. He has helped us NICO folks with low OEM prices (works at an Infiniti dealer) that have helped many of us keep the cars on the road.
Z