They both use NAND chips, SSD features several of these chips, but even accounting for controller + case, there's a big discrepency in price, especially when you consider the economies of scale.
16GB USB = £5 (£0.31/GB)
64GB SSD = £65 (£1.01/GB)
Over provisioning, write capacity, different quality memory, basically about it.
super high quality MLC will do 100,000 writes, and last for donkeys years, decent MLC will do 10,000 writes and still last years, stupid cheap mass produced insanely cheap stuff will start to degrade after a few dozen writes.
SSD, if it dies you get support and a replacement, that includes shipping, testing. Ongoing support, firmwares for increased performance or fixing problems, that is all included in the cost, a usb stick, ship it, never see it again, never hear from the customer, short warranty and most people simply wouldn't bother to get a replacement. Due to the usage few people actually use them very much at all, very few get used in an always on situation, few will be on all day in servers/hot rooms, few will fail.
How much is the cheapest generic ddr3 1333Mhz memory per gb, how much is the highest quality ddr3 3500+.
Essentially we're talking about absolutely different quality components that are also used in a vastly different way, with different levels of warranty, expected use.