[update]I transposed the tape capacity from the article. Now fixed. And I'll take the opportunity to admit there's a bit of apple/orange here: there are records of IBM prices in 360 days, while prices of qualified SSD/HDD today don't get publicly published, so prosumer parts are mentioned.
There's a new paradigm out there, first (in my memory) manifest by Sun with the FS5100 "pure flash" storage (not, repeat not, SSD) about three years ago. EMC has bought up an outfit name XtremIO, which is "pre-revenue". To the extent that a controller can skip implementing filesystem protocols, it will be faster (Fusion-io, for example), and might even be a tad cheaper.
What's been obvious for awhile is that the wholesale replacement of HDD by SSD, as asserted by zealots a couple of years ago, isn't going to happen; the price gap isn't bridgeable. Or is it? The transition from tape drives to DASD (as IBM named it, at least since the 360) also faced a price mountain, or cliff, depending on how you look at it.
Here's a random review of price differential. It calls for a 12 times ratio. Let's go to Amazon, and do a quick check.
Intel 320/300GB -- $505
Seagate 300GB/10K -- $240
Seagate 300GB/15K -- $200
Hmmm. The Seagate prices are all over the place, but in the general vicinity. In any case, not 12 times, modulo specific drives' quality. And, there's the TB cheapy, but we'll ignore them for now.
So, back in 1964, what was the price difference between a 9 track tape drive and 2311 DASD? Amazingly, there's a list. The 2311 was $25,510. You also needed disk packs, as the 2311 was removable. A pack stored 7.25MB. On the tape side, I could find information on 2420 1600bpi. Cost: $54,600, plus another $55,400 for the control unit, which supported eight drives (we'll take 1/8 per drive, or $6,925). 2400 ft tapes, at 1600bpi, could accomodate 40MB.
Assuming packs and tape cost nothing (packs cost more), this is what we get:
DASD -- $3,518/MB
Tape -- $1,558/MB
Difference on the order about what the common sense notion is today of SSD vs. HDD. In time, DASD won out, although (sadly, sniff) largely running sequential file formats. The tape/DASD capacity ratio is sort of where we sit now with HDD/SSD.
So, Mr. Peabody muses that we've been here before, and new and better won. I feel so much better.