Amazon offers the Plextor M8Pe M.2 2280 512GB PCI-Express Internal Solid State Drive for $219.99. Clip the $20 off coupon to cut that to $199.99. With free shipping, that's $5 under our mention from a month ago and the lowest price we could find by $23. It features a PCI Express 3.0 x4 connector with up to 2.3GB/s sequential read and 1.3GB/s sequential write speeds.
Adorama via eBay offers the G-Technology 2TB G-Drive Pro Thunderbolt Drive for $209.98 with free shipping. That's $30 under our January mention and the lowest price we've seen. (It's the best deal now by $59.) This features four 500GB 7200 rpm hard drives configured in a RAID 0 array.