In the UK, the Mark Davies Injured Riders Fund funds independent helmet tests. The results of the 2006 tests are here.
http://www.mdirf.co.uk/enhapresults/Enhap2006REVISED4ppA5v9
Just because a helmet is more expensive than another helmet it doesn’t make it any safer. Just a note that the crush test shows that some of those more expensive helmets are not as good as the lower price helmets.
Please take time to read the link. I have all the studies on the ASTM/SEI ratings and standards as well as the AU/NZ 3838 standards if anyone would like to read them.
If you have had your helmet over 4 years–it is probably time to replace the helmet. The material starts to breakdown and won’t offer the same level of protection that it was designed to give.