Depends on the year, mileage, condition, and the region of the world.

In the US, for a 1990 420 SEL, with 100,000 miles, in good condition, I would estimate around $12,000 to $$15,0000.

A lot depends on the above criteria.

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)