Cheapest is to use an old keyboard and hack it up (that's FREE). Don't do that...
The keywiz eco used to be just under $20USD but it appears to have gotten a little more expensive to build. The only way you will get cheaper is to find someone off ebay/Craigslist who is basically giving their encoder away.
EDIT: Oh yeah, the other way is to get a cheap joystick and hack it up. You might have a tough time finding one with 12 buttons though.
You could try this one if you want to make your life difficult!
http://tinyurl.com/ydu9lx3