Updated! For US readers, I found out that the Doepfer DIY module is available (power supply) from their distributor Analogue Haven in LA. I actually got to visit there back in January and it was an utterly epic analog modular extravaganza. This brings the shipping way way down to a mere $12.50!
I’ve always wanted my own set of synthesizer modules, but didn’t want to spend more than a few hundred dollars, so I reached out to some friends and heard about Doepfer’s DIY module. This is a very bare-bones way to go, but if you have experience soldering and working with electronics, it looks pretty ideal. For €140 (around $156.55 as of writing) you get a temperature-controlled VCO (oscillator, the core component that generates a waveform), VCF (filter), VCA (amplifier/output), envelope generator, LFO (an additional oscillator for low frequencies), slew limiter (often used for portamento), and an inverter (flips a signal over so that positive voltages become negative and vice-versa). That price is just for the circuit boards and cables—you still have to supply any controls (potentiometers, switches)/jacks (i.e. 1/4″ ‘phono’) and power supply plus desired enclosure/panel. They provide a very helpful bill of materials for the former (probably not more than $5-10 depending on how fancy you like your knobs, and a very nice kit for the latter (€120 / $134.12 as of writing). Alternatively, your enclosure could be as simple and convenient as an old briefcase and the panel laser-cut acrylic.
A whole working modular-style synth with rack would total around $300, but the shipping from Germany may add on as much as $125. Still, this is an unbeatable price as far as I know for an entire functioning modular system!