Having studied the SHARP testing regime they are a VERY good guide as to how helmets will protect in the real world in real accidents. The testing is also identical between helmets and so gives an excellent comparison as to how helmets will protect in the same accident conditions.
There are many stories of how epensive helmets have saved people in an accident and also many tales of how cheap helmets have failed. There are also many tales of how expensive helmets have failed in an accident and cheap helmets have protected.
What you need to remember is that the price is largely irrelevant. That comes about due to manufacturing costs, image, sponshorship deals, artwork etc etc.
A cheap helmet achieving 5 stars will protect better than a very expensive helmet achieving only 2 stars in a very high majority of accidents (flukes will always occur that differ from the norm)
The fact that it is a cheap helmet may mean that the quality of lining materials may be lower ie become sweat sodden and stinky in a shorter time and not stand up to washing therefore requiring earlier replacement. It may mean a chin strap buckle may become rusty due to lower quality (although not structurally weaker) metal. The visor may be less scratch resistant than more expensive models etc
The SHARP rating does not consider cost of materials, paintwork, packaging, shipping, image or any other such thing. It is just a DIRECT measure and comparison of protection in identical circumstances.
Obviously a proper fit is key to how well any helmet will perform. The SHARP rating cannot control how well your head fits any helmet.
Please don't think you are getting better protection by spending large sums. Also remember that real world crash and accident types lead to very different head impacts than race track incidents. The SHARP testing fully considers the head impact types with data taken from large amounts of data from real world crashes/head impacts.
To all - get the best fitting helmet you can. Ride sensibly and lets just keep enjoying two wheels while the nanny state still allows it