The lifetime of a helmet should be anywhere between 3 - 5 years, replacing your £100 helmet every 9 months where as a decent £300 - £400 pound helmet should last for, at minimum, 3 years. Baisc maths shows that 36 (amount of months in 3 years)/9 (the length of time you state you keep your helmet for * £100 (The amount you pay for your helmet) = £400 so its a bit of a false economy. In my opinion (and it is an opinion) I would much prefer paying the £400 for a helmet than £100 and replace it more frequently. The other thing to mention is that its never so much about how long a helmet will last, its about how well it will protect your noggin if you have to rely on that helmet. I have seen it quoted else where "If you think you have a £25 head, buy a £25 helmet!"