It seems like what you get for the extra $$$ is better ventilation, removable/adjustable pads, lighter construction. Basically any DOT approved helmet will increase your survival over no helmet tremendously. Once you're wearing a helmet, the differences in safety from one brand to another seem to be much less dramatic, and honestly the research is pretty thin. Most of the info we have is anecdotal.
As stated, getting a helmet that fits properly is key. Having a knowledgeable sales person helps new helmet buyers avoid the common problems of buying a helmet too big, or one that has hotspots that will aggravate over time. Once you know what you wear though, you can save some money by buying on the internet. I had a Shoei X-11 for about a month when I crashed in it. (Ask me how unhappy I was about *that*!) The helmet had a huge gouge across the top, and all I had was a little headache the next day. So I just ordered the exact same one online. (Bought the original from my dealer though.)