As someone who has owned ukes across the spectrum and only moved upmarket since I started, here's what I think. I'll use Tenors so that we can compare apples to apples. And I'll use retail prices to be fair...because with only a few exceptions, I've bought my ukes on sale or used to get more bang for the buck.
$60 Leolani Laminate Mahogany Tenor: Incredible bang for the buck on this one. Got it as part of a $100 bundle at Sams Club: Uke Crazy style case, chromatic tuner/metronome that worked surprisingly well, Aquilas installed, and a spare set of Aquilas included. Seriously a good deal if I've ever seen one. It actually sounded very good for a budget uke because of the Aquilas and I used it throughout college. Lots of memories with this instrument.
$250 Koa Pili Koko Tenor: A big jump in price for a night and day difference. That KPK was an amazing instrument and were very popular here on UU for a while. Solid wood, and a great price. Sounded fantastic with low G Worth clears. When you make that upgrade to solid wood, you do get quite a bit of benefit.
$250 Fluke/Applause UAE148 Tenors:: These were in the same price range but different from the KPK. They did sound good for what they were, and I did love both. The UAE148 probably has the best pickup for the price. And the Fluke has been thrown around while traveling and took it without any complaint.
$450 Pono Koa Tenor Gloss: Back when Pono made instruments out of Koa (I think they discontinued them a while ago), this was a great entry-level instrument especially because it came with an excellent hard case. But to be honest, despite it's fat body, I thought the KPK sounded better. This one was quite a looker though. So this is proof that you can pay more and get a bit of "meh."
$500 Mele Koa Top Tenor: Also overseas produced, and I believe they are set up in HI. You got all the nice solid woods: koa, mahogany, ebony, with top and back bindings..and a subtle rosette. It played like a charm. LOVE this ukulele to death.
$1000: Kamaka/KoAloha/Kanilea Tenors: At this point, you make a BIG jump up in price from overseas-production to Hawaii-made. To be honest, I thought the KPK sounded just as good as any of these...and actually held its resale value just as well. Having owned all three, I think all three were great instruments. But I have no illusions that they were necessarily better sounding than the KPK. They all sure did look better though. And were all set up well from the factory. I had to do my own setup on the KPK.
$1500: KoAloha Pineapple Sunday: What a beautiful instrument. IMO, you're paying strictly for style with this one. It does sound good, but not necessarily better than KoAloha's own Tenor.
$3000: Custom Kanileas: I think when you play in the upper end of the market, I think you're paying a lot for aesthetics and exclusivity. Does a custom Kanilea tenor sound better than a K1-T? Yes. It sounds better, plays better, and looks better. I'd be lying if I said otherwise. But only marginally on the first two points. If I were to quantify it, I'd say the custom is a 10% improvement sound-wise over the base-model tenor. Is premium koa, bindings, a rosette, slotted headstock worth the differential? It's subjective. But you also get some functional benefits: upgraded tuners and a pickup.
So basically, diminishing marginal returns is true. Once you pay enough to get solid woods, tonally, you are probably most of the way to the best instruments available. And I see minimal benefit above about the $3k mark unless you have unlimited piles of money (not a bad problem to have) and want some incredible inlay work. But what do I know, I've never tried that range...but probably never will.