Dell has a Thunderbolt 3 portable 500GB SSD for $440 - $154 = $286 w/coupon code 35OFFBIZ. While most drives such as the Samsung EVO 960/970 get all the glory for their M.2 NVMe super speed, there unfortunately are few to zero portable enclosures for these devices. This SSD is plenty fast according to the spec sheet with up-to 2800MB/s read and 1100MB/s write speeds. It's rather one of a kind at the moment.
For comparison, the top speed portable drive on Amazon I can find, SanDisk Extreme Portable 480GB is limited to 850 MB/s read and 850MB/s write speeds (possibly a USB3.1 bottleneck) and costs $270.
*Clarification: This Dell SSD is a Thunderbolt 3 device, not a USB 3 device, these are a new class of device to the market, hence the Amazon comparison to a USB3 SSD.