Except that's exactly the point: if you do this in your spare time, the development time is essentially "free". The only things that cost money if you do it in

Except that's exactly the point: if you do this in your spare time, the development time is essentially "free". The only things that cost money if you do it in your spare time are the actual materials / PCBs / components / energy. And that is in fact rather cheap, provided you have the necessary skills and tools. 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.