The DAC is really cheap (though they make up for the low price in crazy shipping charges) - there’s only three cheap ICs on the board, with a crystal clock, a few cheap passive components and a couple of cheap gold-plated phono (“RCA”) sockets. Total cost of the parts (in quantity) would be £2 - 3!
The quality of the DAC is excellent - I had one here, connected to a Raspberry Pi. I bought it as a time-saving measure, because I wanted to build a quick web radio for my father’s birthday.
The total cost of my first system:
Raspberry Pi Model B - £23.75 including a “free” 5V 2A wall wart from the Pi Hut (on Amazon)
DAC Module - €29.40 including postage
Aluminium box - gold coloured - Banggood - £4.50
Chrome bezel green LED Power indicator - £0.40 from Banggood.
Resistor to drop the 5V for the indicator - £0.002 (I buy them by the thousand!)
USB Wi-Fi dongle for the Pi - £2.75 from 7-Day Shop
I cut off the mini USB end of the power adaptor lead leaving about 5cm of lead connected to the USB plug, and replaced it with a 2.5mm cylindrical “power plug”. I fitted a corresponding socket to the case, and soldered the USB lead inside the case to the power inlet socket. I did this in case the “free” power adaptor turned out to be no good. I fitted a USB extender adaptor to allow one of the USB ports of the Pi to come out of the back panel, so that the USB Wi-Fi dongle would stick out of the back of the case (online - Amazon, I think, about 50 pence).
I added a power switch and a reset switch - http://www.raspberry-pi-geek.com/Archive/2013/01/Adding-an-On-Off-switch-to-your-Raspberry-Pi using small momentary push switches with chromed knobs (Banggood again, just a few pence).
The “2A” power adaptor turned out to be OK - not noisy at all - so I was lucky there. However, it ran rather warm which worried my Dad, so I put two 7805 linear regulators inside the case, one for the Pi and the other for the ADC. The power adaptor I then used gives 9V DC at 2A (it was for a Casio mini TV) and is linear - it runs cold. I put a 1N4001 in series with the input pin of each 7805 regulator to drop 0.65V on the way in and reduce the work the rugulators have to do, and soldered 100n ceramic capacitors from the output pins to the ground pins on the ICs themselves to keep them quiet.
The whole deal cost around £45, and is flawless.
All my family have seen the one I made for my Dad, and now they all want one!