What is very important is not cost, but fit. All helmets will provide a minium level of protection - they have to by law. Where the extra money goes is mainly on build quality and comfort. (Compare a £60 nitro with a £350 Arai and you can tell where the money goes!).
If you want ultimate protection then you need a minium of space between the outside of your skull and the inside of the helmet - the polystyrene deforms on impact, spreading the force out over time. If there is a gap, it gives a chance for your head to move inside the helmet and suffering some rapid de-accleration (and thus bouncing your brain of the inside of your head - not good). A fifty quid bike show special that fits like a glove with protect you far better than a top flite Arai that dosn't.....
Finally, a word about protective gear - don't fall into the trap of thinking "I;m wearing top-notch armoured levels - I'm well protected". All you'll end up doing is riding that bit more faster and taking a little more risk and winding up attempting to punch a hole in a hedge\wall\truck with your head. Leathers will protect you against abrasion injuries - armour and helmets will protect you against minor impacts only. Any big impact and all your leathers and lid do is make the undertakers job a little bit easier. Most of the dead bikers being picked out of hedges are wearing kit that wouldn't be out of place on a semi-pro race grid.
At the end of the day, the best protection you have is _you_. Get a few miles under your belt, then do some training - the IAM are good for "road skills" and there are a couple of track based courses which are good for teaching riding skills. At the end of the day, enjoy yourself, don't ride like a twat, and you'll be fine
HTH
Andy.
<rubs leg>
