According to http://www.nadaguides.com/

1969 250s:
Low: $2,225 Average:$3,725 High:$5,075

1971 Mercedes-Benz 280SE
2 Door Convertible 3.5
Low: $31,500 Average: $52,700 High: $75,100

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1986 300E 5-Speed 240k mi.