I very much understand the differences in my motorcycle helmets - why the off-road helmet is so very different from the road helmet. But I just do not understand the differences in bicycle helmets. What IS the difference between my $30 Trek and a $200 Giro? It seems they are both just styrofoam covered in tin foil. What am I missing?

I know why I paid over $500 for my motorcycle helmet. Why don't I see the same differences in bicycle helmets? They all seem light weight and well ventilated. What should I be looking for?

I would appreciate input and opinions. I don't want to be penny-wise and pound-foolish here. If there are cogent reasons to spend the bucks I'd just like to know them.