The osmotic pressure you need to overcome to perform desal is directly proportional to the concentration. So 500-1000ppm water that you don't love is a lot less challenging than seawater which is about 3.5% or 35000ppm.
You can do "I want extra pure drinking water from 'normal' water RO" for a few hundred, sure because the pressures are in the range of 20-60 psi. Reasonably efficient seawater starts somewhere around 400-600psi and I've heard of plenty of systems that work at more like 800-1000psi. Different pumps, membranes, membrane housings, piping, all of it. It's a couple thousand dollars plus a fair amount of energy unless you buy even more expensive energy recycling pumps that use the pressure in the brine output to help on the input.
But a few hundred bucks will still get you from either "potable but not great" to fairly pure, or brackish to potable. If you have effectively infinite seawater and power, you can still pump at lower pressure and get pure permeate, it's just less efficient per unit pump and filter lifetime. If you had a super reliable pressure system (low friction ceramic pumps for example, not cheap but last forever) and cheap first-pass filters, you can run it for quite some time. But you're spot on in that you get to a point of engineering-give-a-mouse-a-cookie and it just makes sense to optimize the whole stack.
That combined with reverse flow flushing will probably last many years. And it can all be controlled by a 10 cent microcontroller, one pump, one valve, and a pressure sensor.
I don't doubt that high pressure pumps are bad... but that's just a design issue - there is nothing theoretically expensive about them.
There was a big push towards developing chlorine-resistant chemistries a few years ago, but as far as I can tell, that has fallen into a “researchers don’t know what they’re doing, the plants are already designed around this problem” narrative. Of course, those plants are huge investments, and maybe it’s correct that one wouldn’t be able to take advantage of chlorine-resistant membranes until a new plant was built.
Cellulose triacetate RO membranes have chlorine tolerance, but have inferior chemical stability, productivity and selectivity to polyamide membranes, so it is sometimes used where chlorine tolerance is an issue. CTA membranes are also available in hollow fiber format, while polyamide membranes are essentially exclusively found in spiral wound configuration (some operators want fibers for higher fouling/solids feed). CTA is limited to a much smaller pH window (like 4-6 versus 2-12), and are not suitable for more aggressive cleaning methods, so I’m not sure if it overall provides a benefit versus polyamide RO with more aggressive cleaning cycles.
Free chlorine (technically assorted chlorine oxides like hypochlorite) attacks RO membranes, so now you have to deactivate your reactive species first. Usually UV lamps, you use sun, but now you need UV-clear tubing, not easy.
Reverse flow flushing can be done with the components you mention, that's another $20/50 plus sourcing logistics.
Yes, the theoretical expense in quality pumps is quality. There are tighter tolerances, beefier components, better polymers, and more QA. It all adds up.
It's still all fairly cheap by Western standards, but it's a tall ask for a lot of places that barely have potable water.
Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) but that prices out a lot of developing regions.