As Tay Cough mentioned, I recently started a similar thread. So far I bought a MkIV shell just to try the fit cheaply, but it seems large and heavy even without the headset attachments and I don't think I'll take this option.
David Clarke does a K10 helmet which looks quite good (not too bulky) but which seems a good example of rip-off aviation pricing - though to be fair presumably they have to pay for all the testing but will sell far fewer helmets than if they were designed for motorbikes/skiing/cycling etc...
TBH I worry with the MkIV that it's so large and heavy that I might flip the plane then whack the helmet in situations where I wouldn't otherwise have hit my head, just 'cos it sticks out more. I can also imagine the weight of it making spinal injuries more likely in a rapid deacceleration scenario. On the other hand, padding takes space and more padding provides more protection. I'm no expert in helmet safety but for my own aircraft I'm not sure the large helmets are optimal from a safety perspective.
I'm not going to get much flying in over the next few months, but when I do I'm most tempted by the idea of using CEP in-ear plugs and my lightweight hang-gliding helmet which cost £100.
Her Majesty's Government has seen fit to make piles of ex-military grow-bags available to Ebay sellers, who are moving them on for between £30 and £70. Go-Karters and rally drivers seem to be paying £500 to £1200 for their Nomex outers (new). I'll admit to feeling a bit self-conscious in mine, and given a free choice of colours I may not have opted for either tan or drab. However, immolation doesn't appeal and I'm grateful for her largesse.