A friend and I bought RAM for our aging servers at eBay in 2024. That was 512GB (16x32GB -- we split it equally) of DDR4 ECC 2133MHz, registered. For… $536.86 plus shipping.

Somehow it feels like we got a pretty good deal, in retrospect. (My server has been since replaced, and I can tell you, 256GB of DDR5 ECC was more than that! There's been a further doubling or two of prices since I bought, though. I think I could maybe sell half the RAM and finance the entire server…)