A bunch of good brands make nice complete bikes, even at the entry level these days. You can get something nice from the shop that you'd be able to ride and upgrade as needed for around $300. The more you spend, the better the components will be, and oftentimes you'll step up from a mild steel frame to full chromoly tubing the further up the price ladder.
FBM, Kink, Fit, Mirra Co., DK, Volume, Hoffman, Verde, Subrosa, Haro and others all make completes around the $300 range. As you move up in price you start to get better cranks, wheels, etc.
If your local shop can't help you check out some of the reputable mail orders like Empire, Albes, Staff BMX, and Dans.