The whole helmet thing is smoke and mirrors.
A mate cut open a medium priced AS1698 approved helmet, and discovered a small square of carbon fibre smack-bang in the top.
One of the AS tests in basically a big punch trying to penetrate the outer shell, and yes, the test is always on the top of the helmet - This helmet has been specifically designed to pass the test, rather than actually provide safety for its wearer.
I read a really detailled article in some US road bike mag in the mid 1990s. One of the things that has really stuck with me, is a comment by one of the experts talking about how the Snell test (M95 IIRC) was actually too harsh, and helmets were actually made less safe to pass the test. He reckoned that the safest helmet at the time (I assume it was a road bike helmet) was 'only' DOT approved and had actually failed the Snell test.
No idea how this relates to current standards, but is worth keeping in mind.
I had a pretty big crash about six months ago (six week off work, broken leg, blah blah - could have been a lot worse, particularly given my 'head-first, sack of shit' landing technique).
My $150 RXT helmet did everything that I expected from it: The peak sheared off without shattering (my neck muscles were sore but not seriously damaged), the polystyrene is noticably compressed in one large area, etc.
I really can't see how an $800 skid lid would have improved the outcome in any way, shape or form.
My old MDS helmet was never seriously crashed in. It was about ten years old when I pensioned it off. I cut it open, and discovered that the polystyrene was badly compressed on top of my head - it was less than half of the original thickness, and this was just from use.
From all of this, I've concluded that 'any' an AS1698 helmet is unlikely to make the difference between me picking myself up off the ground, and a worse outcome.
But an old might make the difference.
So I buy a new ~$200 helmet every 18 months, or after a big crash, which-ever comes first.