According to Wikipedia, there were 2,397,514 W123 sedans built and only 199,517 wagons. So neither is really a "rare car," but wagons are about ten times rarer than sedans. Combine this with the practicality of the wagon and it's not surprising they go for more money.
A wagon with working SLS and a 3rd row seat seems to be the Holy Grail for a lot of folks.
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