Thing is, all helmets meet the same basic safety standards in terms of impact resistance -- the only variable factor where safety is concerned relates to fit. If the fit is equal, a $20 Schwinn helmet from Sears is going to protect your noggin as well as a $160 Limar F111. Consumer Reports did a series of crash tests last year and rated most cheap helmets as doing a better job than the handful of high-end racing lids they screened.
Fancier helmets are boasting other benefits -- low weight, better ventilation, precise fitting systems, and radical, streamlined, airbrushed, metal-flaked good looks.
Bottom line? Your only real considerations are, in order of importance: fit, style, wallet. You can re-arrange items 2 and 3 in terms of importance but keep "fit" in the first slot.