I think you are overestimating the difficulty of producing something. Start building stuff as a hobby, and decide that you want to build it as efficiently as possible because you are lazy. You'll quickly realize that building synthesizers and in particular Eurorack modules is easy and cheap if this is what you're focusing on.

What this means: with a Eurorack module, you don't need a case or power supply, since that's provided by the Eurorack "infrastructure". All you need is a "front panel" on your module as well as certain connectors, and probably one of the cheapest ways to build such a front panel is by manufacturing another PCB with the right size and the right holes. You can do that e.g. at JLCPCB, for <1€ per front panel. Building the synthesizer is similarly easy: design the circuit, create a PCB design that uses modern technology and SMT machine assembly, and you can get even "complex" designs manufactured cheaply, to the point that you can even beat Behringer yourself. After all, you don't have to live from selling such modules, and your development time is "free". And the best part: once you have a working design, you can get as many more modules manufactured as you want, for significantly decreasing prices if you manufacture them in slightly larger batches.

To give you an idea, I recently built a small PCB with "transistor ladder filters", but since they are small and cheap, I extended them and made the feedback path voltage controlled and also added a VCA directly after every filter. A PCB with 8 of them, manufactured and assembled at JLCPCB, cost around 30€. For completeness, this was for a 8 voice polyphonic Eurorack synthesizer module, which means this PCB didn't need any knobs/buttons/jacks/displays. Meanwhile Behringer sells a single "Moog VCF" in Eurorack format for >100€.