I installed a GPS system into my '88 Cabbie for about $120.
I am using my existing Gateway laptop (14 inch screen) with Windows XP as the display device, Microsoft's Streets & Trips 2005 for the GPS software, and a Holux GM-210 GPS receiver. I installed a car laptop mount from "mobilelaptopmount", which consists of a Plexiglas platform for the laptop and iron pipe fittings to act as a leg, which includes a large flange bolted to the floor of the car. I located the laptop mount just to the left of the front passenger's legs (next to the center console), and it can be swiveled for viewing by either the driver (PLEASE STOP THE CAR FIRST) or the front passenger.
Total cost, not including the laptop which I already had, was about $120, consisting of $30 for S&T 2005, $40 for the Holux receiver, $30 for the laptop mount, and $20 for the 110 volt power adapter that plugs into the cigarette lighter socket.
The mobilelaptopmount website
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has many photographs of such installations.