For that type of memory, M.2, and the brand/line, the prices are typical.
Since there are so few PS5 games right now there is no rush to invest in a lot of M.2 memory. A cheaper external drive will do just fine for PS4 games leaving the internal stock drive for the few PS5 games that you might have.
Frankly since the PS3 Sony has been a bunch of ****s about their console's storage. The PS5 never should have launch with less than 2 TB. Launching it with only 600 gb of usable storage was one of the most absurd console decisions they've ever made. I've got PS4 games with 100+ gb install files. PS5 games are going to get bigger and bigger meaning at some point I'll probably only be able to fit 3 - 4 games on the hard drive at a time. If I have to uninstall and reinstall every other game everytime I want to play them it makes the PS5 less efficient than consoles of previous generations.
Good job, Sony. Ya dumbasses.