There's about 4 or 5 different styles of 59 Caddy combo car - they weren't made by Cadillac, they were made by custom coachbuilders. The "Ghostbusters" car style was made by Miller-Meteor, and is the rarest and most desirable if I recall right, since that style has been in a couple of films aside from Ghostbusters, and I think people generally prefer the look of that one.
There are a few out there, most are beyond repair, the good ones go for $25,000 + and beyond. Resto costs are obviously way more - things like the glass will cost you an arm and a leg.
The 60-62 cars are much much more reasonable, and while not quite as wild as the 59, are still pretty stylish. You get fins and chrome, at any rate. Some come with the hi-top roofs, some are the classical limo style. You can nab an unloved but restorable early 60's car for sub $6k. After the mid 60s they start getting squared off and go to the worst kind of 70s styling.
For the GB stuff, you can expect to spend another $5-10k tracking down the correct fixtures and fittings, sirens, etc. God knows how much you'd have to spend to nail the interior fittings, that would all be custom work. Because of the nature of where most of the stuff comes from, the GB stuff is damned pricey. If you're interested in Ghostbusters stuff:
http://www.flickr.com/photos/frinky/set ... 198476587/ Replica proton pack I made (and sold, a few of these would get me a D if I had the time)
http://www.flickr.com/photos/frinky/set ... 239023658/ a pre-production Slimer maquette.