I think he's talking about buying two 256 GB's for $99 each and then using something like the Amfeltec to RAID the two 256's together to get a 512GB volume. The four-slot Amfeltec is $400 or so I'd bet the 2-slot is probably $200-300.
Not sure if that'd be economically reasonable for a 512 GB boot card, $200 for the two 256 SM 951s then another 200-300 for the 2-slot Amfeltec..
BTW, got my SM951 256 GB AHCI SSD from MyDigitalDiscount, it says it's a Samsung model, and it was manufactured 2016.09. Weird huh, it's only been out of production for 6-7 months or so?