I think cheap helmets have become a lot better over the last few years, as manufacturing has improved. A $30 giro helmet these days is in-mold constructed, has the same head retention system as the high end ones, and is only down a little ventilation and weight.
The foam itself is the same material. Foam is the cheapest part of the helmet.

I think the fancy skeleton in the expensive helmets is more about adding ventilation without compromising the protection, rather than improving protection.

As some other poster said, the best helmet is the one which fits, and which you will wear.
Try it on, cinch it down, then try to pull it up off your head, or roll it off. Beyond that, if you'll wear a lightweight road helmet, get that. If you'll wear a bmx helmet with a funky design, get that.

I personally like the giro xen- no silly flanges or protrusions, and you can get it in an eye-catching white/acid green color combo.