how much would it cost to put a built in navigation system in a Mercury Marauder

Why? You can not read a road map?

Go to any number of after market sterio suppliers such as Pioneer, Sony, JVC and more. They can incorporate the naviagation system with the sterio head.
Good Luck

The cheapest and most convinient way is to simply buy one at Best Buy. They should be no more than $90 and are just as useful as normal ones and will probably save you thousands. But if you want one built into your car, l wil tell you it will be very expensive because Marauders were not designed to have a navigation system.