Replying to Helmet Talk

I'm in the market for a helmet. Never had one before, but I figure it's the safer thing to do... never know when you'll need one and when you do need one it'll be too late. Kinda like air bags.

So I have a few questions. What are the differences between low end and high end helmets? What causes prices to fluctuate from $30-$200?

What safety features are available? Which are MUST haves?
What are added features that make it flashy? i.e. audio/headphones

What is a low cheap helmet/brand I should avoid?
What is a moderate helmet/brand that is a blend of affordability/good features?
What is the best helmet I can get if I had all the cash in the world to spend?

Plenty of +K to pass around for you guys here.
Thanks!