My local Wal-Mart had the standard USB keypad used for laptops for under $10 on clearence.
As long as its USB I think you could set it up in girder to do whatever you want. If it's ps/2 its possible, but only if you dont plan on using a regular keyboard too.
I have a little project going to play mp3's in my room. I've been using a 350mhz PC with a bulky 14" monitor. I'm taking the guts out and using an old 70's stereo as a case and adding a 7" lcd screen. I have myHTPC and 4am music console for software. For navigation I was going to hack a keyboard and mount some buttons near the screen, but one of these might be easier
Stuff I do while waiting for my carPC parts to get here from Hong Kong ...