There is no such thing as a good commuter bike made in the USA in the $300 range. There are two main reasons.

First, there are no American made bikes in that price range; the American companies have long specialized in expensive racing and mountain bikes and even these use many imported parts.

Second, there are no good commuter bikes from anywhere in that price range, there are few if any at all. Making a bike a good commuter bike is more complicated and expensive than you might think.

I commuted entirely by bicycle for nearly 20 years and am writing from experience. Any bike will get you a few blocks, and any will get you across town once in a while. But if you are really commuting, you need a particular feature set and for many years the market was ignored and the features were not available without customizing.

This article does a good job of describing most of that feature set : http://www.commuterbicycle.com/
and this quote hits the highlights:
"The features to look for in a Commuter bicycle are upright seating position, low top bar, full reflectors, cargo racks, bells, fenders, chain guards, kickstands, gearing for moderate conditions, internal gearing in the rear hub, some mild suspension, built-in generator lights, and nutted bolts instead of "Quick Release" disconnects on axles and seat posts (to discourage part thieves). "
There is a special lock you can substitute for the seat post bolts and I prefer battery powered lights and a higher top bar; but, you can see from the list that this just isn't what you get on a touring bike or mountain bike. My bikes were modified Bianchi and Jamis hybrid bikes (hybrid in this context means intermediate between touring and mountain). Basically, I had the above list of stuff added on where it was missing.

Today a few bikes really are made for commuting, although far more are called commuter bikes while not being at all suitable. One in the Breezer Villager, pictured here:
http://mysite.verizon.net/res6sdo1/commut1.jpg
It looks to me as if all this would need added are panniers and new tires.
But note: it costs just under $800. You could also look at the Jamis Commuter 1 at around $365 but it needs more additions.

One thing that the article overlooks and that I find crucial is the tires. When you are commuting you do not want a flat. Of course, nobody ever wants a flat. But, when you are commuting you really, really never want a flat. You bicycled to work, put in your 8 hour day, and are going home. You do not want to fix a flat, let alone walk it home. There is no sag wagon. And how is the neighborhood you happened to get the flat in? Not so nice maybe? So, there is a simple way to never, ever get a flat: airless tires. Microcelular polyethelene is the proper name. Bike shops won't carry them and always badmouth them. Presumably this is because bike shops earn much of their money repairing and selling tires. Airless tires don't need repair and last much longer. So you buy them over the internet. I use tires from Greentyre in England because I like bright colors; but, there are American companies too. They have slightly worse rolling resistance than conventional tires; but, commuters are not racing to win the Tour de France. They are trying to get to work and back every day without a flat.

One other thing, if you are serious about commuting to work every day by bike. Buy two. One will have to go in the shop from time to time, or simply break down in one fashion or another. Unlike cars, for most people, getting two bikes is not impractical.

http://www.greentyre.co.uk/
http://www.airfreetires.com/shopping/c-1-bicycle.aspx