To be fair, DDR3 is still kinda cheap. It's really only DDR4 and DDR5 that are massively in demand. DDR3 is a bit too old for it to be in high demand by consumers.

I wanted to buy another one and they are like $90 now :(

While DDR3 is cheaper it's still tripled or quadrupled in price over the past two years. I just bought a pair or sticks for an old Mac Pro and it was 4x what i paid just a few years ago.