Replace every 5 years? Really? And while I'm at it, would you trust your grey matter to a $70 Scorpion EXO-400? If not, why and what other brands fit similarly formed noggins?
I'm doing a research project for college right now on helmets. While this question/topic opens up a can of worms (i.e. how much is your head worth thing), the price of a helmet does not mean it is going to protect it any better than an expensive one. There will be a difference in build quality (how solid it feels, seals, weight, etc.), but there are no studies to back up an expensive helmet protecting better than a cheap one.
With regards to safety, you've got the three standard ratings: DOT, SNELL, and ECE. The DOT and ECE are generally very similar, but some argue the ECE is a bit better. SNELL has been somewhat controversial due to how they test - to pass their tests, a helmet generally has to be "stiffer" to withstand repeated hits and because of that, more G force energy is transferred to the head. They've addressed some of this with their new 2010 rating, but it's seems to be the same. SNELL is also voluntary meaning companies do not HAVE to send helmets to them whereas DOT and ECE are government requirements. SNELL will not test modular helmets.
Generally it is simply suggested that you buy the helmet that fits you the
best (fit and budget), as fit is important, and not get hung up over how much a helmet costs. The type of helmet is also important since a full face will protect better than a 3/4 or pudding bowl, etc. It's personal choice, but again, I wouldn't get hung up on cost.