Some of it is the need for precision. Cheap resistors, capacitors etc. can be outside stuff spec by +/- 10%. Which is fine for all kinds of applications, but au

Some of it is the need for precision. Cheap resistors, capacitors etc. can be outside stuff spec by +/- 10%. Which is fine for all kinds of applications, but audio is really sensitive to that kind of variance, so to get the one or two orders of magnitude more precise components you need for it to sound good, you pay a premium. At least this is what I learned trying to save money by building my own modular synth stuff. You'll save some money (labour costs), but it's still gonna easily cost hundreds for a handful of components.