Ken is right on. At a certain level what makes one ukulele "better" than another becomes a very personal matter. In terms of appearance, I think the Hawaiian builds tend to emanate from an overarching island sensibility, at least that is what they may have in common. However, having had the good fortune to play a bunch of them including Kamakas, Koalohas, Kanileas, G Strings, Kelliis, and so on, I can say they all sound a little different. There are innumerable threads on UU, for example, comparing Kamaka and Koaloha. There is no definitive answer as to which is "better," but everyone seems to be able to make an individual choice. The same can be said about the mainland luthier builds in terms of tone and playability issues are concerned; I think, however, the designs run the gamut.
Koa is expensive, but not so expensive that it would in and of itself add a thousand dollars, say, to a builder's cost. Also, while there may be differences in the tone among an individual luthier's instruments resulting from the kind of wood used, particularly when comparing a soft wood top to a hardwood top, in my experience, these differences are not as great as those between luthiers if when comparing ukuleles made of the same woods.
I think there may be a "Hawaiian" sound. I cannot say for certain because I may be projecting a good bit of that. In any event, I cannot attribute it to the koa since most of the mainland koa ukuleles I have played sound more like their mahogany counterparts than they sound like Hawaiian koas.
It is true that the cost of living is generally higher on the islands. I believe it is also true that at least some folks will pay a little premium for a Hawaiian ukulele.