I am particularly sensitive the "best price" per Megabyte of storage, although this is going to have to be adjusted to the price per Gigabyte. This go around, I was pleased to note that 5 Terabyte drives are now becoming available and that they now have the optimal pricing. There are 6 TB and 10 TB hard drives but the extra storage space is much higher priced than what would be expected. For example, a 10 TB hard drive is much more expensive than two 5 TB drives.
All such considerations are relative. I did some research into the historic cost of computer memory and in 1995, the standard RAM memory in a desktop computer was 8 Megabytes, hardly enough to operate a computer today. The 1995 large capacity hard drive was 9 Gigabytes and cost $2,399.00. (See http://www.jcmit.com/diskprice.htm and http://www.relativelyinteresting.com/comparing-todays-computers-to-1995s/) Today, a Seagate Expansion 5 TB Desktop External Hard Drive USB 3.0 (STEB5000100) is $149.63 on Amazon.com. You can compare this to a Seagate Backup Plus 8TB Desktop External Hard Drive with Mobile Device Backup USB 3.0 for $299.99. The largest hard drive I have seen at Costco is a 4 TB hard drive for about $119. But I haven't seen a 4 TB hard drive on sale at Costco recently in our local store, but the 4 TB drives are still available online.
The simple answer to the question in the title of this post is that prices are always coming down as new technology is developed. In case you need a reference, here is a chart from Wikipedia: Terabyte, that shows the differences between the different terms used for memory capacity.
Most genealogists would never use all of the capacity of a 4 TB hard drive in their entire life, but even though the drives may have that capacity, this does not mean they will last a very long time. Here is a link to an explanation from Seagate about the real failure rate of hard drives: "Hard disk drive reliability and MTBF / AFR." What this means in practical reality is that your drive may fail at any time. For this reason, I maintain two and sometimes three backup drives. I presently have my main internal 2 TB hard drive and three external backup drives. I also copy my primary backup drive periodically and give a copy of the backup hard drive to one or more of my children.