For one, price has little correlation with the quality of usability.
Really?
What inexpensive items can you think of that nevertheless have excellent usability and aren't clear derivatives of more expensive items of the same type?
People who know what they're doing cost money, period. If you want a good user interface in a commercial product, you have to get it designed by someone who knows what they're doing when it comes to user interfaces, and that costs more than having it designed by someone whose expertise lies with some other area that is necessary for the project. More precisely, it is not uncommon for software, especially, to have its user interface designed by the same people who designed and implemented the rest of the software, since the necessary expertise to design and code the software is much more fundamental to the success of the software than is user interface expertise. While someone who doesn't know a whole lot about user interfaces can put together a poor but functional one, someone who doesn't know a whole lot about coding can't design and code a functional piece of software
at all.
Indeed, as regards user interfaces, it's even worse than that. A good user interface not only adheres to general usability principles, it's designed so that the
target users can properly use it. That means that the usability expert has to not only have expertise in user interfaces, he must also have expertise in the domain the product is intended to target. As regards oscilloscopes, it means the user interface expert must understand the features of the oscilloscope and how they are to be used. Merely being good at designing user interfaces isn't enough.
Many very expensive products have terrible user interfaces.
Yes. But that alone doesn't break the correlation. While many very expensive products have terrible user interfaces, few inexpensive products have good user interfaces (but see below, as there are clear exceptions to that). Good user interfaces are more easily found in expensive products than in inexpensive ones. Look at Linux and Windows, in contrast with Mac OS, for an excellent example. Apple products command a premium in part because Apple spent the money up-front on usability, and focused so much on it that usability became their signature trait.
(For example, the first generation of BMW's iDrive became famous for how terrible a UI it was.)
Yes. How long did it continue to have a terrible UI? The first generation of anything isn't necessarily a good metric to use for determining whether there's a correlation between expense and quality of the UI.
If anything, you often see the opposite, that expensive (i.e. non-mass-market) products have terrible UIs because they're made in such small numbers that the manufacturer can't invest in big UX projects.
Then how is it that Windows, a mass market product with many more seats than Mac OS, has a much worse UI than Mac OS?
There are products that are relatively inexpensive which have solid user interfaces, but those tend to be mainstream products for which a poor user interface would be a major competitive disadvantage, or which are heavily regulated (e.g., avionics), or which have to be competently implemented because a screwup on the part of the operator could cost lives (e.g., cars, for which the controls for the major systems are almost always well-designed).
In the end, it comes down to the ability and willingness of the manufacturer to spend the needed extra money on user interface design. That money
will increase the cost, so the expenditure has to be justifiable somehow.
While there's no question that the DS1054Z's user interface has problems, exactly what would be the justification for Rigol to spend the extra money on people who know how to design solid user interfaces, given their target market and general market strategy? How would you expect them to recover their increased costs as a result?