Obryn
Hero
In fairness, I don't think we have any clear indication what the demographic for a product like this would be, because we haven't seen it yet. Your cost/benefit equation is completely imaginary, since there are zero actual numbers to base it on. Honestly, the only third-party setting I can name for 4e is Wraith Recon - and that was a pretty clear rush job. (And it was Mongoose.)But it would be difficult to develop, and you are one of a probably small demographic interested in the final product. Cost/benefit does not look very good for a 4e Arcane Evolved.
When/If Earthdawn 4e comes out, that'll be a much better measuring stick.
Until we see someone produce a high-quality and unique setting, and then actually market it, I don't think we'll have any idea how many potential purchasers are out there. Much like with Cirno's pronouncements about the overlap of 4e fans and Dark Sun fans, you have no evidence that the number of potential purchasers is small.
Also, keep in mind - I'm simply saying what a product would have to be like in order for me to purchase it. It very well might be a flop - again, I don't have any evidence, either. Sadly, we're past the days where companies could rush out any old thing, sell it for a tidy profit, and then work towards better stuff. (And even the companies who produced some good stuff were mostly working on things that I and others didn't really want or need, and really didn't market or publicize it very well at all.) The entry bar is higher, and so is the risk.
-O