Amazon has Audio-Technica's popular ATH-M40x professional monitor headphones on sale for $79, a price drop of just over $20. They normally sell for close to the $100 mark and only rarely drop any lower than this; when they do, it's generally for just a few dollars more.
The M40x headphones offer some of the best bang-for-your-buck you can find for sub-$100 headphones. They use 40mm drivers, a circumaural design for passive noise-isolation, 90-degree swiveling ear cups for one-ear monitoring, and professional-grade material for extended comfort. The Wirecutter considers them a runner-up in their category on the best headphones under $200.