"Uh, Wrong Forum, Comrade"

Perhaps one day I will do some Eurorack modules.  Cost, as you mention, is multi-faceted, and time is a HUGE cost indeed.  Eurorack has the advantage of having a decent range of DIY kits for modules, as well as many published circuit designs, which shortcut many time-consuming steps in the process.

I can't speak towards a VCO, as I haven't yet made one.  But I can give a breakdown of the power supply module I just made, in estimated terms.  Here's the scratch-built module design process:

1. Conceptualize the module: determine the desired functions of the module, sketch some possible front panels, search around for what others have done/include in their designs that you may also like. Time depends on how much you care to dream about the module you're designing. (1-1000 hours, but usually 1-4)
2. Start circuit design: this overlaps with step 1 a bit, as limitations of board size, component availability, etc. may limit what the module can do.  Examine design tradeoffs, read through datasheets, find similar circuits. Time depends on circuit complexity, unique component count (ICs, for example, each have their own datasheet to review and glean inspiration from; the more ICs, the more time), ICs chosen. (1-10 hours, usually 1-5)
3. Test, adjust, and finalize circuit design: test the circuit on a breadboard, troubleshoot issues, redesign circuit sections as needed. Time depends on circuit complexity and circuitry experience. (2-20 hours, usually 2-5)
4. Layout PCB: render circuit designs into an ECAD software, then place components and run traces per the circuit design and front panel design.  Time depends on the same as 3.  (1-5 hours per board)
5. Etch PCB: toner transfer, etchant shielding application, etching, cleaning, continuity and isolation testing. Time depends on number of PCBs needed in the module. (2 hours per batch of etched boards)
6. Drill PCB: put hundreds of tiny holes (for THT component leads to pass through) in the PCBs exactly where they need to go.  Time depends on circuit complexity. (average 1 hour per board)
7. Solder boards: get all those electrical bits where they need to go.  Time depends on available tools, circuit complexity, and soldering skill. (average 1 hour per board)
8. Make front panel: wherever it makes most sense, create the front panel. (0.5 hours)
9. Test module: power up module, sniff and stare for magic smoke.  apply test signals as needed to ensure module is functioning.  Troubleshoot physical connections when the module doesn't work.  Cry a bit.  Repeat. (1-5 hours)
10. Test module in system: mount module in rack, pray, turn on rack, make a test patch. (0.5 hours)

For my power supply module, I was fortunate in being able to skip step 3, as my power circuit was lifted directly from the TL7905 datasheet and basic circuit principals floating around in my head.  The I/O circuitry followed suit. It contains 3 PCBs, which were etched in one batch and all passed their first electrical tests.  An estimation of that module's time for completion is:
1. 3 hours
2. 1 hour
3. 0 hours
4. 3 hours (simple boards and simple circuits)
5. 2 hours 
6. 2.5 hours (simple circuits, only three boards)
7. 3 hours
8. 0.5 hours
9. five minutes
10. 0 hours (I don't yet have a system to test it with; it IS the system so far)

... for a total of about 15 hours over the course of one week.  

All of these given times are overestimations, preferring to state too long a duration than too short.  


For a DIY module that I already have one of, I'm able to skip right to step 5.  So if I wanted a duplicate power module, it would "only" take about 9 hours or so to make.  

This is a LOT of time, and if evaluated at California minimum wage ($12 USD), this module would cost me $15 in parts and etchant + $72 = $87.  Here's the rub, though-- for me, and perhaps me alone, I can find the spare hours to make such a module far easier than I could find the spare dollars to purchase some equivalent (in this case of custom modules, there is no equivalent on the market, but perhaps Eurorack may be thought of as the same).  I also enjoy the design, build, test, and play process very much.  That being said, I simply could not afford to sell scratch-made modules like this at a "reasonable" AEM-level price-- I would certainly have to move board production to a PCB manufacture company.  

A VCO module would perhaps take a similar time to complete.  Many VCO circuits exist out there, with many different architectures (the TW VCO, I believe, runs off a hex inverter IC, and op-amp-based VCOs abound) to suit your personal flavor.  I do not have access to an oscilloscope, so it would be a tad bit trickier to test than my simple power module.  But it could be done, in a very rough-and-tumble "good enough for my ears" sort of way.