I've spent untold hours, as I suspect many of us have, reading helmet reviews/tests, threads like this one, ad copy, etc. and I have seen no evidence between helmet cost and the ability of a helmet to do it's ultimate job of protecting the head. In fact, I have seen plenty of evidence that indicates NO relation, the primary example being the Motorcyclist helmet testing article of a few years ago where the helmet that scored best in terms of protection was one the least expensive at under $100.
What price does seem to be related to on a regular basis, although there are exceptions, are the peripheral functions and features such as venting, graphics, fit, finish, number of features, lower weight etc. Some of these features are truly functional and offer a measurable benefit while others are simply preference/subjective. However, most helmets by the majors are doing well enough in these peripheral areas that an $800 helmet is hard to justify when a $200 helmet comes close to or, in some cases, meets or beats the same mark.
In my case, I have yet to justify more than $200 on a helmet. I have only owned three daily/primary helmets:
1) Late 80's Nolan N42 (full face, not the recent 3/4 model)
2) HJC CL-14, purchased new in 2006 for just over $100, now my backup
3) HJC FS-15, Elbows II graphic (Ben Spies) just purchased new in 2009 for $173 plus shipping
I was considering Arai, but after being sized by two different "experts" from a premier Arai dealer and getting two completely different recommendations on both size and shell, I simply couldn't justify an extra $300 or $400. As it turns out, the HJC FS-15 actually fits me better than any of the Arai's that I tried. Go figure. ???
Of course, whatever specific full face helmet one chooses, the more important choice is that of choosing to wear a full face helmet in the first place.
Best of luck!