I can chip in on this topic: I have assembled all in all 6 AdLib replicas. Two for me, one I donated to the 8bit-guy, which ended up in one of his videos. The rest I sold off. The PCBs were 20 EUR each, with heavy gold plating and thick copper, made by German manfucturer. You can get them MUCH cheaper from somewhere in China, so there you can save a lot, probably down to 5 EUR per PCB without gold plating and thin copper layer.
The components were about 35 EUR per card, including the OPL2 and the DAC. I ordered high quality components from Mouser. You could have saved a few Euros on the potentiometer, plus much cheaper capacitors and doing away with no IC sockets. Probably going down to maybe 20-25 EUR per card.
This gives you minimum 25 EUR material per card. Or maximum of 55 EUR for high(est) quality components.
Then comes the soldering, plus drilling holes in the bracket, testing. This took me the better half of a workday (in my spare time obviously). Assuming 4 hours of work, given a rather low compensation of 10 EUR per hour you end up with 65 to 95 EUR per card. Then eBay wants 10% off of your price. Hence 80 USD is quite the fair price I think, depending on the quality of the materials this guy used... 😀
All in all, I encourage everyone to try and build such a device yourself. It IS immensely fun and educational. Disadvantage is that you always order at least 3-5 PCBs, which is the minimum order at all manufacturers. And short of wasting all the PCBs you will have to build more than one! 😀
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80486DX@33 MHz, 16 MiB RAM, Tseng ET4000 1 MiB, SnarkBarker & GUSar Lite, PC MIDI Card+X2+SC55+MT32, OSSC