The only reason I buy slightly upscale helmets (Bell Metro, last year's model, $40) is that I have a big head and nothing in the department stores will fit.

As far as looks, dude, we're adults riding bikes. With a cheap helmet, dork factor=90. With a $250 helmet, dork factor=87. Not worth it. The only people who will even notice are other cyclists, and most of them take one look at a commuter rig and write you off anyway.