BHP Safety Products via eBay offers the Howard Leight Electronic Ear Protection Earmuffs, model no. R-01526, for $37.89 with free shipping. (Altatac vie eBay offers them for a mere 10 cents more.) That's the lowest total price we could find by $10, although we saw them for a buck less last summer. These ear protectors automatically block noise above 82 decibels and feature an external audio input and built-in microphone.

Update: The price has increased at BHP Safety Products via eBay; however, Altatac via eBay still offers them for $37.99 with free shipping.