Well, there are a couple of different standards that helmets can be tested to: a helmet carrying a Snell rating should offer a tad more protection. Whether that's ultimately important or not is another question.
IMO, I go for fit first. Any helmet will have a hard time performing to spec if it's wobbling about on your head.
All helmets sold in the US meet minimum ANSI safety standards. The lower end helmets have less venting as noted before. Less vents more material possibly more collision protection. Just an assumption on my part.
Good link. A study was conducted with helmets in the 10 to 20 range campared to helmets in the 1 to 2 hundred range. All helmets offered the same protection against impact. The higher priced helmets used a higher density foam to compensate for the extra venting.