We asked for it and our readers delivered. [Klulukasz] left a comment pointing to this diy RFID reader that was a final project in 2006 for a class at Cornell University. It is well documented and includes not only a schematic and code, but an explanation of the design considerations used during the build. The project uses an ATmega32 and the parts list priced out at about $50 at the time. There were plenty of responses to the RFID spoofer post pointing out that there are readers available for $40, but we want the fun of building our own.
A bit more vague with the details but no less interesting is this other simple RFID reader design. Thanks to [Chuck] for his comment which pointed to that link.