The Seagate Ironwolf 8TB drive I've kept an eye on for several months hasn't changed much in price in the UK, it might have even come down in price. US tariffs?
OP is referring to used drives, i.e. server pulls.
Quite obviously it's supply and demand. Some huge quantity of 12TB HDD were decommissioned a year ago (and earlier) and thus prices were very low.
The current AI boom revolves around massive deployments of Nvidia GPUs. But prior to that, there was steady expansion of conventional data centers from the cloud operators going back many years. I'm guessing there was a huge surge during the heart of the pandemic (2020), and a lot of spinning rust was deployed at that time. Presumably, most of those drives have since been retired.
The other factor is that SSDs have gotten cheaper. So unless you need massive storage capacity of hard drives, cloud operators just don't need as much spinning rust as they used to.