Buying a cheap helmet is definitely ill advised.
When you look at the comparison between cheap helmets and "real helmets" and top of the line helmets, it comes down to FAR more than ventilation and weight. Higher end helmets will typically have an internal skeletal frame that the impact foam is built around. Cheap helmets use cheap impact foam won't have well functioning fit "tensioners" and sure as hell won't have any internal structure.
Its not the initial impact that kills you, its the secondary one. The more impact your helmet can resist, the safer you are.
Not only that but how much is your brain worth? How much did you spend on your education? Enough to justify spending $200 to have that extra insurance when you're staring into the headlights with no where to go?
~DC