Over 500,000, that's really mainstream...
It's niche, but a bigger niche ('niche' being a relative term). I don't think a game like Muramasa would ever get sales like that, even if it were marketed very heavily, unless perhaps it was the first of it's kind. That's why they don't give it mainstream marketing in the first place. It happens all the time, Madworld was heavily marketed, and what happened with that? There is such a thing as being limited in appeal, and quirky games like Muramasa are exactly that.
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.