A 480GB Crucial M500 is slightly cheaper per GB than a 4TB spinning drive right now. I think the 960GB SSD is as well.
That comparison is meaningless because a 4TB is at a premium price. If you think you need 500GB, use should compare a 500GB HDD with an SSD (480GB being close enough). I can get a 500GB 7200RPM SATA drive for about $50. A Crucial M500 is about $120. The SSD is 140% more costly or 2.4 times the price per GB.
Even that comparison is a poor one. Really this all depends on your mission.
If all you want is an OS drive for your Chromebook/etc, then you want to look at the cost of 16-32GB of SSD and that is as cheap as any hard drive you could get in that size configuration. The SSD is an obvious choice here.
If you want to store your video collection and your options are RAID HD or SSD, then you don't care how big the individual drives are so you look at price per GB. That usually will end up costing $80-110 for the hard drive in any year - the only thing that changes is the size. That will get you about 2-3TB of HD, which is about 4 cents/GB. Compare that to something like 50 cents/GB for SSD. Clearly if you're storing video the SSD is a really bad choice.
When you look at HD prices you need to stay close to $100. You don't save much money by cutting capacity below that, and you don't get much capacity by spending more than that. I'm not as familiar with the dynamics of SSD, but I imagine that they too tend to have a sweet spot, and it only makes sense to compare apples to apples.