If $600 is your price range, then the Costco roadbike is a good option. You might be able to find a used name brand bike for a similar price or some previous year's stock at a bike store, but frankly I think you're lucky to get something in the $800-900 range. You also might spend months looking instead of riding.
I do find that "Fitness bikes" - flat bar road bikes, which make excellent commuters do get discounted more than drop bar road bikes. If you don't need the multiple hand positions that's another alternative.
I'd be wary of the hybrid and mountain options. Disk brakes and suspensions are rarely both cheap and effective.
Whatever you do, enjoy your ride. There's still nothing like the freedom of a bicycle. Like H.G Wells said, "Every time I see an adult on a bicycle, I no longer despair for the human race."