This is a very variable question and just about impossible to put a dollar amount on. New, remanufactured or used?
Due to the precise tolerances and special tools involved, individual replacement pieces aren't recommended.
Which turbo? There was stage I, II and III.
What is wrong with the old one? Seized, noisy or blowing oil?
Which engine? If it's the Turbo II, it's a conventional Chrysler turbo engine. All kinds of things could make it run rich: O2 sensor, excess fuel pressure, bad coolant temp sensor, etc. I think the car should support the normal key dance error codes. I don't see how a bad turbo would cause black smoke (as Bob mentioned).
There was also a 2.2 with a Maserati head - very expensive if it ever needs head work.
If the turbo was completely locked up, you could have drivability problems and possibly black smoke (?) but I agree that you're going to need more troubleshooting. Blue smoke could be any of a myriad of problems; blown rings, PCV issues, leaking valve seals-- OR a blown turbo seal. I think the owner may not necessarily know what he/she is talking about. In this case, I would call buying the car off until I had seen it smoking and done the key dance on it myself.
Or you could get a rebuilt turbo thru one of the TD vendors for $500 with no core. If the turbo is really bad, that's the route I would go. Also check some of the vaccume lines. If some are broken that could cause the car spit black smoke out. I had that problem with my 87 GLHS, black smoke at idle, finally replace every single vac line and it went away.
It should still be drivable even with a locked up turbo, if that's what the problem is. It would drive pretty much like a regular EFI car would feel like. You can try to turn the turbo vane wheel with your finger with the inlet hose off and the engine off.
If it is oil smoke or rich mixture would have to be determined.
It is tough to change the turbo with the head on the car as it is on the back of the motor. You may want to pull the whole head and turbo assembly to service the turbo. If that is in fact what has happened here.