Okay so, roughly, we are looking at selling 500-1000 per state. Of course, some states are a lot higher, and other a lot lower, but it averages out. Now from my experience, I would wager that maybe a 100-200 of those are steampunk fans. There will also be some that buy simply because they are RPG fans, not because the like steampunk. I'm would be one of those since I don't usely buy RPG books to play, but rather just to steal ideas from them. So the RPG buyers are maybe 100-200 also. Of course there are those who know nothing about the books, but plan to give it to someone as a gift. We'll say that there are maybe 50 of those. That leaves about half who just have a passing interest but are intrigued by such things as the back teaser and cover illustrations or a recommendation from a friend or other acquantance such as a good store clerk. As you can see there are a lot of casual shopper in the equation which is how a lot of merchandises gets perchased.
Course, the flaw with this thinking is that it assumes RPG buyers are more like regular book buyers. I couldn't say if the typical RPG steampunk buyer puts more research into their purchase than a normal book buyer, but it still supports my basis that perhaps having a breakthrough product is not necessarily the most crucial thing. A mediocre product will likely fare as well as anything else on the shelves simply by getting the casual shopper.
Course, a bad/good economy will also alter the equations. Fads also make a big difference. So the question really is: "Will steampunk be the next big thing in RPG?"