Update [11/24]: Sold out at Amazon, but still at Best Buy.
$50 price drop! Amazon has the Crucial MX500 4TB 3D NAND SATA 2.5" Internal Solid State Drive (CT4000MX500SSD1) for $320 with free shipping. Best Buy have it for the same price. Has 560mb/s and 510mb/s sequential read & write speeds, respectively.