Howdy,
Pacmanfan here,
I suppose a flatpack is a psuedo good deal if you don't want to construct one from scratch. Another good option is to go to a local auction and pick up a dead cabinet for a great price. I just got a beautiful 19" generic jamma cabinet with everything except for a board for 20$. That includes the control panel, coin door, 19" monitor, isolation Xformer, jamma harness, etc...
I'm not sure what a flatpack includes beside the wood. You'll probably end up spending a bit on hardware, latches, coin door, monitor, etc...
I've also has success in going through the phone book and looking under "Amusement Devices" to find local arcade operators. Most ops have a few cabinet they'd like to get rid of. I've picked up quite a few nice cabinets that way for 50$ (including a really nice dig-dug.)
Also, you'd better brush up on your soldering skills. Head off to Radio Shack and get yourself a soldering starter kit (solder iron, tips, solder, braid, snips, etc...) for like 20$, and practice soldering and desoldering on an old MB or something.
just my .02$
-PMF