At one time, the major technical difference was the way the thin plastic shell was attached to the styrofoam body. A cheap helmet would have the shell taped around the circumference. If you pressed down on the thin shell, you'd feel a gap at many points where the shell did not touch the styrofoam. This would result (after several months of use) in the thin shell cracking and flaking off, and the whole helmet looking pretty ratty.
An expensive helmet, on the other hand, used "in-mold construction," meaning that the thin plastic gets bonded directly to the styrofoam. There's no tape, and no gaps. This results in a helmet that resists pieces of plastic flaking off.
However, nowadays, just about all helmets use the direct-bonding approach. So the difference will more likely be style (coolness), number of vents (coolness), strength-to-weight ratio, ease of adjustment, etc. So your cheap, hot, unstylish, and heavy helmet will likely last just as long as the expensive one. I'm sure there are values to be found. Me, I just wait until the Bell Volts from last year go on sale online. I know exactly how they will fit, and what size I need (another advantage with the top-line helmets is that they are widely available and easier to find online). But if you're not sure, you'll just have to try them all at the LBS.
Luis