IMO if you find one with a good service history, under 200k, in great shape, working A/C and no apparent issues; $5k or less would be a good price.

The wiring harness is easy enough to check, just look at the wiring at the temp sensor behind the upper radiator hose. If it's falling apart, expect to replace that. ~$450 if you DIY.

In LA/San Diego they seems to fetch at least that much; significantly more in Seattle where the bio thing is still huge.

Geographic location has a lot to do with it. I'm always jealous of those finding 05-06 CDI's for reasonable prices, in So Cal they're usually 10k or so for a nice example, in Seattle people want $15k+ for cars either 200k+ AND/OR salvage titles.

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1995 E300 Diesel
~300k