Once, just once, I would like to see someone say “why is this so expensive, it’s just […]” and then build one themselves, and bring it to market, for that lower cost. Go ahead, just try.
In practice you'll probably skip the "and bring it to market" step though, because usually you'll start such a project because you wanted to have a specific mixer/synthesizer/sampler/…, and once you have it on your desk, the next project usually looks more interesting than spending the extra time to sell whatever you just developed.
I mean at the extreme case, if you can tolerate 40ms of latency and stuttering, a Raspberry Pi and some USB soundcards can do it all. The actual power needed is reasonable and ADC/DACs aren't that insane.
A Teensy 4.1 connected to an ESP32 could probably do most of this, if you wanted to do $15k of software development in your spare time and were really good at DSP.
My playing is already sloppy enough to not be affected till maybe 15 or so.... but it would clearly be different with a pro!
Or maybe they just have some good IO. It doesn't seem to be CPU limited exactly, it's just the built in sound device doesn't like it if you turn down the block size too much. USB cards are a bit happier.
Are you including those probably custom knurled knobs, buttons and sliders in that guesstimate too? Because those are the sort of things I'd expect to bring the cost up here. Heck, that beautifully finished aluminum case with its beveled edges alone is good chunk of money.
Since this whole post was about a Teenage Engineering hardware module originally, which has a strong focus on the case: if you own / have access to the necessary tools already (e.g. lathe, milling machine, …), it's a bit time consuming but not exactly expensive to build something similar. Sure, e.g. aluminum costs money, but together with the electronics I'd be surprised if you really spend 1200€ (price of the TX-6). Of course in that case it wouldn't really scale if you wanted to produce more modules in the end. For completeness, the TX-6 page specifically mentions the ADCs and DACs used. If you check the (current!) prices on mouser/digikey, they are ~5€ for the 4ch ADC chip (you'll need 3), ~15€ for one of the DAC chips and ~13€ for the other one. Then maybe add some fancy STM32 microcontroller for another 15€ and you essentially got the digital part already, excluding wireless connectivity. You'll still need a few analog components, the faders, the switches, connectors, and a display (probably the most expensive part). Feel free to guesstimate what the complete electronics and aluminum case would cost, but as I said, I'd be surprised if you get even close to 1200€, even if you need a 2nd revision of the electronics.
There are also various examples of people building e.g. clones of Moog synthesizers like the Minimoog (including keyboard, wooden case, …) in their spare time, for less money than what the original Moog synthesizer would cost.
Also most of the disagreers are also ignoring the "bring it to market" step. Even if you built something of equivalent product design as TE, building one for yourself is vastly different than doing a product run where you expect each product to be the same. PCB design and manufacturing is relatively easy and inexpensive, but all the plastic/rubber forming/molding is much trickier and significantly expensive. Getting past the manufacturing aspect, there's the whole marketing, sales, and distribution aspect, which aren't anything to be sneezed at either. Seriously a lot of work goes into bringing a product to market that DIYers building single devices or very tiny runs of devices don't even consider. While TE might not be doing Behringer-level runs of products, their runs are definitely bigger than a lone DIYer could ever handle by themselves.
(Note: This isn't to say that TE's products are necessarily better for everyone than something they could make themselves (or buy for less from a different company), of course. After all, the world's musicians got by just fine before this mixer was released. I just wanted to chime in and explain some of reasons why TE's stuff costs more than what you might think you could make yourself.)
No one is forcing you to buy this product.
It's not quite the same thing as the TX-6, but if I were to go to the substantial effort of making a small mixer product marketed to people like myself, that's the sort of competition I would be intimidated by, not Teenage Engineering.
If it's going to sit on a desk and be used all the time, a Behringer 1222USB is $250 when they aren't running a special sale. It has more channels, some usable effects, and you can add an iPad Mini with some synth software to handle the rest of the functionality. So that's half the price, and you get to play with the iPad whenever you aren't making music; or it's about 1/6 the price of the TX-6 without the synth.
Think YouTube podcast where everything on the desk is sleek and apple or apple-like. This fits in in a way the Behringer doesn’t.
I wouldn’t be surprised at all to find YouTube videos with this on the desk but obviously not being used - where the actual dirty work is done offscreen.
Portable music devices aren't that expensive these days. Take a look at what SunVox can do on old hardware, or maybe the Polyend Tracker, which is a hardware solution that costs less than half of the tx-6 and doesn't kill the eyes on a toy display.