But, really, it like talking about a good, great and magnificent apple. You know a bad one but after that it's ... an apple.
Apples are a good analogy. If you never had a magnificent apple from a local tree, or at least the farmer's market, and only know the watery greenhouse kind of apples from the supermarket, or the fruit imported from some place thousands of miles away and that was pulled off the trees before even ripened, you won't know what you're missing. But once you get used to "real" apples, the difference between those and what you get from the shelf in a mall is huge.
With ukuleles there is a tipping point somewhere, that sweet spot where price and quality meet. I'm not exactly sure where the point of diminishing returns is, and it probably depends on each player, their expectations, skill level, financial situation, and what they aim for. And also what they have been exposed to before, like with apples.
For me personally, bling is fairly irrelevant, and, for example, inlays don't matter to me at all, no matter how artistic and time-consuming they are. I can appreciate their beauty, but they wouldn't enhance the enjoyment I get from an instrument. It's the same for fancy wood grains: There are beautiful woods out there, and I can marvel at them, but they don't add value to my experience. For me, it's all about playability and sound (and sound is relative -- I dig the KoAloha's signature sound, while others would call it brash and loud) and you can get that from a plain instrument like an Opio.
Extras, like pretty wood, a hand-applied finish, decorations, etc all add to the price and potentially the experience, but to me they don't impact whether an instrument is good or magnificent. For others, though, those aspects do matter and do make a difference. They certainly increase the status value and quite possibly the resell value, too. Like with sneakers, you also pay for the brand name sometimes. There are also functional features that raise the price, like an arm rest, which are also meaningless to me personally, but definitely do benefit others and are valued by them.
So anyway, there is probably no objective answer to the question. For me personally, I think the soft spot is between $500 and $600. After that you get diminishing or no returns (depending on what you value), and before you have to make some sacrifices. For koa, which I'm not sure I could tell from other forms of acacia in a blind test, you have to add another $400-500 to that.