I'd say that, as you go up in price, you hit different feature levels (generally speaking).

Taped-shell becomes molded-in shell;
Cheap or difficult-to-use adjustment straps become simpler, ratcheting mechanisms;
Universal sizing (Child and Adult) becomes multiple sizing (Adult S, M, L, maybe XL somewhere);
Plastic-and-foam becomes plastic-foam-carbon-Kevlar-etc.

I tell people to go with molded-in-shell helmets that fit comfortably and securely. I trust the molded construction to retain its structure better after the first hit in a crash, expecting that it'll still be on the head for the second and third hits. If it's comfortable, it's going to be worn more often.

After that, it's frills -- lighter weight, more venting, etc. Remember that a lightweight, well-vented helmet needs extra reinforcement just to pass the same standards that the thick, heavy, barely-vented $20 Wal-Mart brain bucket has to pass, so that's when the pieces of carbon and exotic tweaks like a molded-in Kevlar reinforcing web are used. Stuff like that adds cost -- and marketability.

Not totally true. The sticker only means that a minimum standard has been met. It says nothing about helmet designs that exceed specs.

Some higher end helmets have imbedded CF or a ballistic fabric weave that holds the helmet together in case of a multiple impact crash. Cheap helmets may take one hit and fall off.

The retention and adjustment system is important and the more expensive helmets employ more advanced systems.

Venting and weight are also considerations effecting price. The manufactuer wants pro teams using their best product so looks plays a factor as well. That is for advertising/sales, not safety.

That's what I think too.

Unless you're looking for the absolute cheapest thing that will get you by, I don't consider helmet cost to be a very significant factor. Essentially you're talking about around a $150 price range for something that you'll use every time that you ride for around 5 years. That's $30.00 per year. Might as well get one that you like.

FWIW, over the years, and in this order, I've had a Giro taped-shell helmet (forgot the name), Giro Xen, Giro Pneumo, Bell Metropolis, and Specialized S-Works (got an insider deal on it ;)). The one I wear the most, because it's the comfiest, lightest, and most-vented, is the S-Works. You could argue that, even though it's the most expensive, it's been the cheapest in terms of cost-per-mile or -per-ride.

I'm keeping the Metropolis as a rain/winter helmet. I gave away the Pneumo to a neighbor, and I stomped on the old taped-shell helmet and broke it just to see how strong it was (it was pretty strong). I kinda have to keep the Xen; I tell people to never buy a used helmet because you can't know its history.