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I guess it's just a question of semantics. You said, "There are no materials which are cheap enough on Mars and expensive enough on Earth to every make freight plausible." Typically, when you said something is cheap or expensive somewhere, you are referring to the price it costs to purchase it. When you say something is cheap on Mars it seems strange to me. Everything is free on Mars, since as far as we know, there's nobody up there claiming they own it already. The thing that is not free, would be to extract these materials. We would have to spend a lot of currency here on Earth to purchase needed materials and motivate people to do the necessary work. Maybe it would be better to say, "There are no materials which would be cheap enough to extract on Mars and valuable enough on Earth to every make freight plausible."
I'm okay with that wording. By the way, this has been gone through pretty exhaustively with respect to He3 on the surface of the Moon. There, it's well known that it's worth shipping it back to Earth. It's a fusion fuel which pretty much does not exist here. And because the Moon has no atmosphere and a low gravity, means of practical return-tank-to-earth exist.
Because of the heat/cold cycles of Lunar day, and the complete vacuum, there may well come to be various scientific experiments on the Moon for which it is desirable to return samples to earth.
There could also be scientific experiments on Mars for which sample return to Earth was plausible. Not that it would be economically efficient, just that some guys here wanted those samples at any cost.
But in terms of traditional materials production, metals, plastics, for example, there is nothing so rare or valuable that it could be returned to Earth for profit. I can envision some electronics being left out in the open on Mars, and then a solar storm occurring, and it being desirable to return the circuitry to Earth for detailed examination (learn how to make it better, right?)