$400 seems to be the going price for a rebuilt/remnaufactured unit.
If you pull off your intake hose (before starting the engine, so it's nice and cool and not running!!) you can actually see your turbo compressor inlet. You can move the rotor to check for wear. It should spin freely and have virtually no play in it. In a severe case, you might see metal-to-metal wear. If there's any appreciable end-play in the shaft its certainly due for replacement.
I'd schedule it for sooner, rather than later. The cheaper prices on turbos assume that you have good parts to return. If the impellers and followers (fixed parts) are scored, they might not be reusable and could cost you more money.
Another option might be to get an entire turbo and manifold from a later model car, liek a 940. They're easier to service as well. The Garretts were replaced by Mitsubishis startign around 1989.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 221K, 88 744GLE- 202K, 91 244 181K, 88 244GL 145K