The most expensive component required is a logic analyzer. While I
use the Saleae Logic it may be too expensive for casual hobbyists ($150).3 An
alternative, OpenBench Logic Sniffer, is considerably cheaper ($50) and probably
adequate. My choice was dictated by the needs of a teaching laboratory
where equipment takes a terrific beating – the exposed electronics and pins of
the Logic Sniffer are too vulnerable for such an environment. An Oscilloscope
might be helpful for the audio interfaces, but is far from essential.