I'd say $150+ or so. Maybe cheaper, if it's one of those acrylic ones. There is a possibility of GH numpads coming down the pipe, I think, but based on gh60's progress....
There was a whole conversation on this here:
http://geekhack.org/index.php?topic=49242.0 , plus you can find others if you look. I'm not so sure what numpad does this properly. I do know the Realforce numpads do, but they're not cheap either
I believe the Cherry 3700 outputs correctly. And if not, it's programmable, if you get the correct version, so you should be able to change it?
At the end, the Poker II is programmable, right? Can't you modify it to add a numpad layer? I just did this to my HHKB.