IMHO: the key to a cheap commuter is buy local. Lots of deals, all sorts of mid-line bikes pop up on craigslist. Buying local and the right size, with fender/rack braze-ons, in excellent shape is more important than brand name. Most of the sexy names will command a premium and make the bike a target.
Waiting for a used/excellent shape bike (local so you can inspect it) is key. Doesn't take much wrong to kill and dreams any of low cost. Sometime you can find a bike that's been tuned up with new tires/seat for cheap. Who cares what brand if it rides right?
I've converted a Ross Mt Whitney, Schwinn Cimmeron and Giant ?? to single speed commuter/errand bikes for myself and my 2 sons. They ride just fine and there's not too much cash invested. Tires and nice seat/grips/fenders should cost more than the bike (IMHO).