You could always try some junkyard scavanging and see if you can get a deal on the Navi parts from a wreck. But, would you know enough to be able to retrieve ALL the necessary parts for the retrofit? I wouldn't.
Another option, just get an aftermarket unit! It'll save you a lot of money, a lot of installation time, and a lot of headache.
There are several aftermarket ones that look pretty good. I think they range around $700 to $1,000 for decent units. Sam's Club had a Magellan (I think) for $750.
www.gpsworldsupply.com has some inexpensive products. I purchased a factory refurbished Garmin E-Map for $115. I bought it three years ago, and have never had a problem with it. I still use it now, on my motorcycle.