Very cheap, if you have a compatible car ("VWs, Audis, SEATs and Skodas").
There was an article in Circuit Cellar Ink about an ODBII interface; it looked like it wouldn't cost much to build. October 2000, issue 123. "Routine Checkup—An Automotive Diagnostic Scan, by Dan Harrison"
You can buy the interface, and/or d/l the software: http://www.ghg.net/dharrison/
If you can find the article and build your own, the .s19 file for programming the micro is at: ftp://ftp.circuitcellar.com/pub/Circ...arrison123.ZIP
I just can't figure out where the ODB interface port is on the one-barrel Holley 1904 carb on the 223 Ford six.