The wiring and "light box" are quite simple, I believe. The main cost is the screw-in fitting. It's a very tight tolerance item, with two spring-loaded metal contact pieces that are electrically separated, embedded inside a round nylon housing. One contact triggers light A, the other light B. When both are lit, the internal flywheel tang is dead center on the two contacts so they both are triggered. Nifty piece. Would be quite hard to replicate, IMO.
Price in Europe, in Euro $'s, is $151:
http://wwwsg.daimlerchrysler.com/Projects/wi/cda/etpl?ETNR=W617589082100&from=query&etpl_lang=01
Order from speed-autoteile.com (in Germany) and you can probably get it for +/- $200 USD or so - I think.
