Demand and reputation, as stated, are the biggest factor. If demand goes up, but production numbers don't follow, price goes up instead. Plus, names like Sadowsky are very well known in the professional bass world, and that has a large influence on price. If people know your name and are willing to pay a premium for items with that name, why not charge the price they are willing to pay. It's good business. Plus, a high price tends to give the product an air of exclusivity, as not everyone can just drop 4 grand on a bass. There's a lot of basic economic powers, and even more psychological drives at work here. We humans are weird critters, and we really value exclusive high dollar goods like American Laklands and Sadowsky's. Part of it's a material, concrete example of self-worth (a guy that owns and uses a Sadowsky is probably more successful than a guy that owns and uses a MIM Fender). Basses are like cars for a lot of people.
Also, there is, on occasion, a job that requires a quality J or P bass, and Sadowsky's and Laklands are recognized and well liked.
And then there's the special cases, where there really is no comparable product, like those from Carl Thompson, Ken Smith, or Vinny Fodera. Each one of them builds a product on a similar price level, but in terms of playability, construction, and design philosophy, they are wildly different from anything that came before them. Carl was the first to build the Piccolo bass, and the first to build the modern six string bass. Vinny, is, I believe, the reason that all of these other fancy single-cuts are around. Ken builds all of his instruments with such exacting detail that many luthiers who otherwise wouldn't have are forced to constantly improve the quality of their instruments, lest all of them be left behind (this is, in the end, almost a futile struggle, every Smith I've been close enough to inspect has had absolutely perfect construction). Guys like these can, and do, charge a heavy premium for their instruments. The people who are lucky enough to be able to pay do, and not only do they hand over many thousands of dollars for a bass, they do it with a smile on their face and thank the man for the privelidge. And why not? These fellows are artists in the classic sense, not just luthiers.
As to the last question, I could see some Sadowsky's that are hand-made by Roger himself being of better build quality than some Valenti's, but on the whole that's more about experience, and Roger's been doing it for longer. The gap can and will close. As for other people in Roger's shop making them, it's probably a much closer race. Also, the love the person building the instrument puts into it cannot be overerstated. If you love what you're doing, and you love what you're building, it shows through in the finished product. That's why guys like Carl and Vinny are at the very top of the game, they don't just do it for the money, they do it for musicians, not because they need it, because WE need it. That's a labor of love, right there.
And thus ends what is probably the biggest brown-nosing post I've ever made.
