Do you have all the parts laying around to build said bike? For a beater a $250 frame and $80 (sweeeet) fork seems like a lot of money to me. If you need to buy most of the drivetrain, wheels, attachment bits you could end up with well over $500 in this bike.
If you do need lots of parts why not go with something like a Kona Lana'i (super low end at $450) and swap the fork for a P2? Also the Kona Smoke would be a sweet beater. Personally I picked up a craptastic Norco a couple of years back for $230 at Blocks. It's on it's second drivetrain and I changed a lot of the parts to stuff that I had collecting dust (wheels, saddle, stem, cranks, etc). I call it the uber beater, you can tell all the parts that are steel as the are now Ferric Oxide color (rust!) from two winters of Regina riding!