Given that labor on Mars will be scarce, Mars colonists are likely to make high 6-figure incomes. Also, given the opportunities to establish new businesses on a new world, many are likely to become multi-billionaires.
My point is that it doesn't have to at first. (Some European North American colonies didn't have a real economy at first, either.) Right now, there are people paid to do research in Antarctica. (Not paid as much, but if the current international legal climate towards colonization of Antarctica changes, this will change.)
> Mars would have no net exports to Earth, and no real wealth
In the context of a solar-system wide civilization, this is patently false. Mars would be tremendously wealthy for reasons that are analogous to the wealth of the North American continent. Mars has many of the things that Earth has (exploitable ore deposits, water, and potential for agriculture) but it has them in a much shallower gravity well, so basic physics would dictate that Mars would dominate trade over Earth in such a context.
Whichever civilization realizes that Earth is just a local maxima for human potential and dominates the colonizing of Mars will dominate the human history of the solar-system, just as English speaking western civilization dominated the 20th century through the settlement of North America.
My hunch is that many Chinese scientists and engineers understand this very well, and this will result in a concerted effort by China to colonize Mars. So what I'm saying, is that there will be initial wealth transfer from Earth, fueled by nationalistic competition.
If you're working someplace remote, and you're not getting paid above market, then you're doing something wrong. (Hell, I've heard there are glorified clerks in remote places in the Aleutians that make near 6 figures for basically sitting around.) Given that some Mars colonists will be very rich, and that many governments and companies will pay you to do engineering and research there, Mars colonists are almost certain to have high salaries.