Depends on how far you want to go with it. If you want an aftermarket touchscreen NAV on a permanent display, it will obviously be more expensive than say a garmin
Cost depends on many factors. I would visit a few stereo shops to get an idea of the damage.
For the factory NAV option, again it is not cheap. Not 100% sure, but I believe you need more than just a "NAV unit". You probably need a new head deck stereo, NAV interface board, and a different wiring harness. The NAV uses a hard drive to store the data and I think they put in a smaller one for models without NAV. Plus, I think the hard buttons are different between the NAV and non-NAV cars.
Sorry for the bad news...but it is an expensive option for good reason.