DealExtreme: $29.99 ELM327 USB Vehicle OBD-2 Scanner Tool
It uses a clone of the ELM327 chip and is only $30.
From what I understand it has a RS-232 to USB converter chip onboard so you should be able to pull the data off as RS-232 with a little hacking.
Considering that it already has the OBD connector, dc/dc converters, etc. It might be a little cheaper than building those parts from scratch.
Of course one could always just hook it up to a Netbook, Palm Pilot or Pocket PC and use it as is.