It's worth what someone will pay for it.

300SE's and SEL's are a great deal because most 126 buyers are looking for the V8 variants. I personally am not a big fan of the V8's as they are mostly around to appease the American market. Look at most of the early classic MB's and you will see them powered by inline 6's including the "legendary" 300SL Gullwings. The V8's do cost more to maintain and are not really "that" much more powerful. A head gasket job or anything requiring the removal of a head costs you twice as much on a V8. For example my S320 is a 230hp inline 6. The S500 of the same vintage is 275hp. The difference is 45hp with a lot more fuel cost and service costs.

A well maintained car is something to pay a premium for. But someone made a valid point in a previous comment. Being the lowest cost model you may find a higher number of not very well maintained vehicles.