B&H recently had an HGST 10TB on sale for $299, but it's back to $319.
It's not your imagination, HDD prices have been stuck for a couple years now. Hanging a little above $30 per TB - more for premium models. Larger drives have been introduced, but unlike in the past, they aren't a better deal. They just cost more. And existing sizes don't drop in price. I've been holding off upgrading myself for two years now, waiting for a correction.
There are technical reasons why bigger drives are harder to do today. But that doesn't explain why existing sizes fail to drop. I believe it has more to do with reduced competition. There are only 3 real competitors today (WD, Seagate, Toshiba) as HGST is wholly owned by WD.
SSDs were putting pressure on the smaller HDDs, but flash demand went through the roof, and SSD prices stopped declining ... 2 years ago. They've even climbed by about a third since - we're in a global flash memory shortage. So nearly all the competitive pressure vanished, and HDD value stopped improving. Draw your own conclusions.
As to deals, the 8TBs are showing signs of improvement. Limiting things to 7200 RPM drives, you can get 8TBs on ebay for as low as $200 ($25 per TB). You can improve things a little further if you buy 5400/5900 RPM or an SMR drive. These drives may not have a manufacturer warranty though, as they may be grey market (imported from Europe/Asia) or shucked from externals.
I won't touch SMR myself, you can't use it in RAID. Or swap it in as a system drive. It's limited to whatever you bought it for, and cannot be retasked during a later upgrade. So it becomes a disposable purchase. The 5400/5900 drives are better, they can be RAIDed to stretch their lives if HDD prices remain high. But they still tie your hands somewhat, by being poor system drive choices.
My advice. Hold off as long as you can, buy as little as you can, until prices correct. I'm hoping for movement this Black Friday. If not, the SSD market will slowly correct as more capacity comes online. And Toshiba (newest guy on the block) is slowly gaining market share. Eventually they'll all have to stop taking profits and compete.