AVR-Chat

The cheapest 'keypad' is also the most elaborate.  The standard PS-2 
keyboard. They all have internal microprocessors that use (now days) 
about 5-10 milliAmps.  The 4-wire interface is standard (+5, Gnd, 
data, clock) and there are programs in C (check Atmel's application 
notes for the AVR) and Assembler (in this group's file listings) 
written for AVR ready to adapt to the application.  You can get 
MiniDIN6 connectors from old obsolete motherboards.