As long as the $35 helmet meets CPSC or Snell standards, the biggest difference is $165.

All helmets sold in the US must meet CPSC standards.

Typically, the more expensive helmet will be lighter (often by 100 grams), and have better ventilation. Cheaper helmets also tend to come in "one size fits all" designs, and have some tension adjustment to the retention system, so more expensive helmets, which typically come in different sizes, may fit better.