Normal HDD costs go up very little as drives get larger. For example, an average 1TB drive costs a little under $90, 2TB costs a little over $100, and a 3TB drive costs close to $150. For HDDs, the cost per GB goes down as the number of GB goes up.
SSD costs don't work like this: a 128GB SSD goes for $120ish, 256GB goes for $250ish, and 512GB drives get up to $600. The cost per GB goes up as the number of GB rises.
What is it about SSDs that makes them so much costlier as they get larger?