Blades should be cheaper then USBs for many reasons. Redundancy - If something breaks on your blade the whole blade is out but it's not likely 30 USBs break at once. Divisibility - You can sell as many or as few USBs waiting for top price while they hash away.
Its the other way around.
USBs need USB hubs
USBs need *disputed* usb fans to cool
USBs can't exhaust the heat as easily in a farm
USBs take up WAY WAY WAY WAY more space
USBs require a host computer
USBs require a lot of cabling
Blades are under warranty, USBs are sold with redundancy rather than a warranty.
Blades are modular.