I think one cost advantage to an internal gear hub is that when your chain and/or steel sprocket are worn out, replacing them is all of $20 or so. This is likely going to happen less often than you'll need to replace an 8 or 9 speed chain and a whole cluster or a few gears out of the middle of it.

The other cost issue is that in terms of shifting smoothness, etc, the comparison has been made with a brifter system. Well, you're looking at considerably more money to brift than to be internally geared, and you have to have a front derailer to match the gearing range of the hub - that doesn't make the weight a wash, but it's close. (maybe there are 11-34 cassettes, I suppose, but I'd personally feel wary about parking an uber long cage derailer where it might get kicked)

The way I see it, it costs about $225 bike-store retail to have a 8-speed hub and shifter. Decent rear hub + 8-speedcassette/chain + rear derailer + friction shifter is cheaper, but if you go to 9-speed bar-ends and a front derailer and the extra money for double or triple crank over a single you're probably spending about the same money. I would furthermore guess that with low-end brifters the increment in cost is approaching twice as much.

Just this weekend I'm building up a Karate Monkey with an 8-speed hub. We'll see how it goes! If I like the bike I might make it my late fall commuter.