At that price point your options are going to be limited, but it won't be impossible to get you on the road with a reliable bicycle, with which you can commute.

I'd say that you have three really good options:

1) Keep monitoring your local CL (only buy a used chromoly steel frame bike)

2) Go to www.bikesdirect.com or www.nashbar.com
( Last day 4 free delivery at Nashbar is today! ~ 1/20/12)

3) Go to your friendly Signature Schwinn LBS and checkout some Single Speeds.

a. Coffee 1 Single Speed ( Dirt Trail Ready) ~ $345

b. Willy 1 Single Speed ( Dirt Trail Ready) ~ $330

c. Racer (Not 4 Dirt Trails) ~ $300

Good Luck!

- Slim

PS.

WALMART
(* Only consider if someone can check for proper installation of parts)

If all else fails and this turns into an emergency, then go to Walmart and actually begin to mount various bikes for fit.

I will only vouche for the GMC Denali and the Schwinn Varsity. Don't buy any bike at that price point that has a suspended fork. You only want a Rigid Fork!

None of these Walmart bikes, are dirt trail bikes!