TLDR version: The best in the world at what they do are perfectly within their rights to charge whatever they please. People will either buy or they won't. The long-winded version is below.
My take is this. Get them now, while they're cheap because it won't be long until you're paying a much higher cost for top quality customs. Ukuleles are still unreasonably considered 'novelty' instruments by much of the uninformed world. My guess is top quality customs will sky rocket in price over the next decade as the rest of the world wakes up to the fact that ukulele's are serious instruments to be respected (something most of us here already know). The top uke luthiers in the world fetch a little over $10,000 for their absolute top of the line models. These are instruments with hours upon hours of intricate inlays woven into the instrument. These people are absolute geniuses and are the absolute best in the world at what they do. Workmanship of the same quality, with the same amount of time spent, and marginally greater materials costs would fetch tens of thousands of dollars if they were building guitars (Granted, the guitar market is probably harder to break into at the moment due to the greater number of established guitar luthiers).
Is the luthier that the OP is referencing one of the 'absolute best in the world?' I don't know, but the title of the post is about 'hand made' ukes in general, not a single luthier.
If you think custom ukes are priced high, google top of the line, custom mandolins. $25,000 easy. One quick google search of custom guitars brought up a Monteleone at $65,000. Ukes are cheap, even the most expensive ones. Not a single one of us can judge if someone charges too much for their hard work and expertise. Only the broader market can determine that. I don't mean to say that $4,000 isn't a lot of money. This is all relative, of course, and not everyone can afford, or is willing to pay, $4,000 and up on a musical instrument.
To put a different spin on it. Let's think about a brand that does both broad production and customs. Why would someone pay $5,500 on a 2019 KoAloha Black Label that sounds marginally better (granted that is just my opinion) than a KTM-00 that they could get for a little over $1,200 bucks, especially knowing that, at the rate KoAloha innovates, much of what you'll be getting in the Black Label may be introduced into future iterations of the KTM-00? Because for many, it does indeed sound better. Because it plays smoother. Because it's unique. Because it was built by the best of the best that KoAloha has to offer. Because, for some, it's beautiful beyond measure.
I'll step down from my soap box now.
@WestyShane - As a former cycling addict, Amen, brother.