LightInTheBox offers the Moon Unisex Bike Helmet in Black/Red for $16.99 with free shipping. That's the lowest price we could find by $3, although we saw it for a buck less last May. It's available in sizes M and L.
Note: This item ships from China and may take two to six weeks to arrive.
The planet will thank you, your local (overwhelmed) landfill will thank you. Even your budget will thank you.