I bought a new helmet at the weekend, £20 reduced from £30. I went out with the intention of paying a lot more but when I tried on the £50 – £60 helmets they were very uncomfortable and a bad fit. On closer inspection they were made of the same material as the cheaper one and made to the same European standard. There was no considerable weight saving and the venting looked to be adquate on both. The only significant benefit of a dearer helmet was a flashy seatbelt type strap fastener but that can't really justify an extra £30 – £40 can it? Am I missing something here? I mean it's my head! I'd gladly pay more for more/better protection but this doesn't seem to be the case.
Full face or XC helmet? Didn't think XC helemets had seatbelt fasteners?
Anyway, double-D fastners are genraly considered better anyway as you can't forget to do them up tight, and can't jam the seatbelt into the cloth part of the socket by accident and ride off. IIRC race/track day helmets have to be d-ring fastenings.
Fit is the crucial thing. Some helmets may exceed the basic standard but thats not tested.
Unfortunately for me an expensive helmet ws the best fit for me.
I replaced a damaged helmet with one that cost around twice as much still only around 60 new however I got it for 35.
It is significantly more comfortable, easier to adjust on the move and much much much cooler/better ventilated. I would say they are worth the extra and last years on clearout seems a good way to get one.
better vents genrally in tha tthey work and have been tested for flow rather there ar ejust some holes in it.
i have a £20 and a £70 specialized one and can tell the difference when it is warm otherwise no real difference
everones head is different, wear what is comfortable for you...not neccesarily the dearest..... also ive said this b4..the 'specialized' helmets are quite a 'small fit' and you have to have a 'specialized head' to fit the helmet (so the bike shop says ! 
(slightly) lighter weight requires more time spent on design optimisation; more complicated tooling is required for higher end helmets with multiple outer shell components or dual density polystyrene bodies; kevlar frames that you cant see inside the polystyrene etc. All adds to the £££
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At the end of the day it's still doing the same job like you say.
Above and beyond the required standard, fit, quality of workmanship, durability, research and development costs of improved designs etc.
Well I've just come back from my first ride with the new helmet and I'm happy. Sure another helmet may have better ventilation but mine's ok and it's better than my old lid that I paid £45 for 3 years ago. I reckon the important things are fit and comfort and it's fine in both those areas. I guess there's only one way to really test how good a helmet is, here's hoping I don't find out.
For those that are wondering it's a specialized align. Now, what to spend the £40 pound I saved on.......Hmm!