For example on Newegg a vanilla 7200 HD from Western Digital, at 1tb, will run you $50. The lowest cost 1tb SSD will run in the mid $300s. A solid 7x difference.
In less than two years we'll see consumer retail 1tb SSDs for $150 to $200. What's actually going to happen, more than a violent price plunge pushing 1tb down to $50, is capacity will soar while prices fall more reasonably than the article indicates. So you'll be buying 5tb SSDs for $300 in three or four years. The difference is the friction the SSD makers will insert into the market, when it comes to falling prices; ie the difference between what they could sell them at, and what they actually will sell them at. The race will be to higher capacities first, then as capacity saturation nears, the bottom will fall out of lower SSD storage tiers on pricing.
Is there a point at which they simply cannot get cheaper?
Those new "shingled" Hard Drives are 8TB and pretty cheap. However, the hard drives have severe reductions in speed and are really only for archival purposes. IIRC, there's a path for 10TB, 16TB and beyond if you're willing to put up with the weaknesses of shingled hard drives.
Granted, SSDs are moving from MLC(2-bits per cell) to TLC (3-bits per cell) which reduce speed significantly without actually selling any "more" hardware. So both sides are "cheating" extra capacity out of the same hardware.
If you want to keep the same high-quality specs that the 4tb hard drives have (speed, reliability, etc. etc.), I don't think there's a valid upgrade path at the moment. There's some research to push the capacities beyond that while retaining the speed of current hard drives, but they're not ready for commercial use yet.
But the same is true for SSDs. 16nm MLC Flash might turn to 10nm or 8nm as process technology improves... but that's maybe a 2x to 4x improvment in capacity. TLC gives another 50% boost.
After SSD, what technology will take us to 100pb drives at $300? Is that even possible, while keeping the small size of the drive? No idea. It's going to require some amazing breakthroughs.