The HTC One M9 is HTC's new flagship, but there are still plenty of M8s floating around out there. Best Buy is looking to unload some stock today with a steep discount. You can get a new M8 for AT&T, Sprint, or Verizon for $299 off-contract. This deal is valid today only, or until supplies are exhausted.
It's common for last year's model to get cheaper when an OEM moves on to a new flagship, but the HTC One M8 has a lot in common with the M9. Both devices have a 5-inch 1080p screen, aluminum unibody chassis, and front-facing Boom Sound speakers. The M9 has a Snapdragon 810 instead of the 801 in the M8, but that might not be a good thing for the M9. The 20MP camera on the M9 is also rather disappointing, but in a different way than the M8's Duo Camera.
When you figure in the price differential, the M8 is still a great purchase for HTC fans. $300 for a new device is downright reasonable. These devices are carrier locked (except for Verizon, of course) and will get OTAs only when the carrier sees fit. At this point, the M8 is probably done seeing significant updates anyway.