Shouldn't it be the other way around. A bicycle helmet will protect your head in a fall and only weighs a few ounces and has plenty of ventilation, a motorcycle helmet uses a pound of material to do the same thing and cost five times as much. As a more direct comparison a full face down hill helmet will protect your head(sixty mile an hour crashes in rocky terrain) as well as a motorcycle helmet with better ventilation, half the weight and a third of the cost.
You can also buy an cpsc or snell aproved helmet for less than $40, and it will protect you as well as a $120 helmet, it just weighs more and has fewer vents.