Above a certain level, money makes no difference to safety (and some would, rightly or wrongly, argue that sometimes it hinders). What you get are improvements in the following:
1) Comfort: If you’ve ever put on, say, an Arai (assuming your head fits), it’s like having your head in a pillow. It’s great.
2) Aerodynamics: How easily does your head cut through the air at speed? This often affects comfort and neck/shoulder fatigue on longer trips (30 mins+).
3) Noise: Sometimes this is a result of better aerodynamics, but for the most part, the top-tier helmets are much quieter than cheapos.
4) Ventilation: This is especially true of race-caliber helmets, but usually the higher-end helmets move more air through the helmet to cool you off (though most of them have vents that can be closed for cold-weather riding). On hot days, full-face helmets can get VERY hot and stuffy, so ventilation is great.
5) Fogging: Some higher-level helmets are better at resisting visor fog-up, but it’s really hit or miss as to which do their job. Also, a pin-lock visor can just about get rid of fogging issues regardless of helmet.
6) Gadgets and gizmos: Features like one-button drop-down sun visors, modularity (conversion from FF to 3/4 face), bluetooth, etc. will drive up price.
Aside from the fact that these features may affect your comfort and visibility, which in turn might affect your safety, none of these will protect you more when your head actually does hit something.