It's even seems priced well at $1.15 @ 1k
It seems like a job for a microcontroller to me. How about an ATtiny4 that's under $0.50 in q100 ? (PIC 10F and Freescale RS08 are also available in that price range.)
Note that avr-gcc does not support tiny4, and it's a bit "odd." Not having to write and maintain firmware might be worth double the price!
Edit: note that the ADK2012 schematics that was published recently (and is supposedly the first detailed glimpse of the Arduino Due) has a PIC10F "Secret Knock" device on it! (connected to the ARM "Erase" pin.)