Marketing and hype (like much of cycling gear) based on erronious belief that:

1. A more pricey helmet works better for protection
2. That $80 of extra engineering goes into a $100 helmet over a $20 one. (If this was true why do they keep re-engineering the top of the lines models. Why not sell last years design, etc. - (OK, not a good example, but gets you thinking)
3. That is costs 5x more in materials and manufacture. These are mass produced and use the same materials (in fact less on the expensive models) maufacturing/materials costs are probably in the sub $5-10 range.

Al