Cool. What would your preferred form be? I haven't decided how I'll offer it.

A. I send you my blank PCB and a link to print the enclosure on Shapeways, with a bill of materials listing all the necessary parts, and you order and solder the parts yourself? I've designed it so far so that all soldering is all through-hole 0.10" components. I would sell you merely the blank PCB, and everything else you order yourself, but I'd give you links to everything.

B. I send you a populated PCB with all the circuits and connectors soldered in, but you have to buy the Arduino board and the BetzTechnik board yourself, upgrade both of their firmwares, and plug them together. It would still require you to solder some pin headers onto the Betz board, but that's pretty easy. I'd supply the correct pin headers to make sure they mate up with my PCB.

C. Fully assembled, all parts sourced by me.

Ballpark prices below. I don't have solid numbers for everything yet. For any items I'm supplying myself, I'll add a small margin of a few percent to help cover my costs and time. A few bucks more if I'm doing any soldering or assembling.

The board price for my PCB would be my unit cost for the low-production-run PCBs from Pad2Pad, so the individual cost per board is still kinda high, my blank board would probably be in the $40-$50 range plus shipping.

The enclosure from Shapeways would probably be about in the $60-$80 range plus shipping depending on whether you pay extra for a rush production. I don't know the exact price since I haven't finalized an enclosure design yet. That will occur after I print my next PCB revision and test it.

The BetzTechnik boards run about $60.00 plus shipping (including the WT32i chip).

The Arduino Mega boards run about $20.00 plus shipping.

I haven't priced out the circuitry and components for the remainder of the stuff. I was buying components in bulk so I'd have to look them all up. Figure somewhere in the $40-$80 range for all the circuits and connectors? Not certain.

Finally you'd need to be comfortable with doing this to your player, since I'm not really to accept other people's rare unobtanium artifacts into my care:
https://github.com/tfabris/BlueGigaEmpeg/blob/master/I2S%20Wiring%20Modification%20Photo.jpg

I'll do a more detailed post in General when I've got something closer to final and can post a good video demo.