There are basically two different types of helmet material, PC (polycarbonate) and GRP (glass reinforced plastic). When new they have similar impact characteristics.
However PC helmets need very careful handling as any minor impact can induce stress cracking - which may not be seeable. They are also adversely effected by solvents - eg gasoline.
A GRP helmet is far more durable because of the nature of construction. Interwoven glass fibres are not over effected by impact and solvents will do no more than affect the paint job. They will take minor knocks.
I doubt that there is much difference in the safety qualities of a 2,500 Penguin helmet and a 15,000 Shoie. With the Shoei you are paying for higher quality construction, better materials and above all, design comfort. As far as safety goes, probably 5-10% safer. All these helmets are 1,000,000% safer than the 200 NT helmets bought at the roadside.
The most important aspect has been mentioned before - a good tight fit. One of the worse causes of injury is rotation of the head during the accident which can cause permanent brain damage.
As an aside - would anyone agree that the introduction of the “must wear a hat law” was commercially, rather than safety driven?. Surely any responsible government would have ensured that all hats had to comply with an appropriate international standard.