I think word of mouth has really caught on now. A friend of mine is obsessed and saw it almost once a week for while (student rush made it affordable), and from what she said it's only been 50-60% filled in the fall, but it seems to be much fuller now. It certainly every time I saw it.
Like Lizzy said, it has low running costs because it's only 2 actors & 3 musicians and it's in Ken Davenport's own theater so they don't have to worry about rent for the theater so much.
I hope it'll keep running, and will get at least nominated for a few Outer Critics Circle and Drama Desk Awards, that would put it on the map more, too!
As for off-Broadway grosses, I've never seen any anywhere. My guess would be because so much of it is non-profit...