Hello, kids! Captain Obvious here...
I don't suspect that people are going to buy this solely based on performance, because you can likely get the same performance from Tom's $2500 SBM. The intended buyers are obviously paying for the fact it's a boutique build. The price also goes up because these guys want to get reimbursed for the time spent engineering it and the materials used. (Granted, the materials aren't that expensive, relative to the time and knowledge needed to do this type of custom-build.)
I look at boutique builds just like I do anything "boutique" whether it's cars, motorcycles, or fashion-related. The sellers work on a low volume / high margin sales model and aren't concerned that other sellers are giving equivalent performance at a fraction of the cost. It's not all about performance for the seller nor the buyer. It's about exclusitivity, or bragging rights/ status symbol, or "beauty in the eye of the beholder", or any combination of the three. And more often than not, the price tag for such items is top-tier.