This is an Interesting Question

Today's hard drives have a MUCH longer MTBF (Mean Time Between Failure) than those of yesteryear. They are, however, asked to work a lot harder and they are mass-produced in third-world countries, which was not so much the case in the '80s when they were primarily used in stationary mainframes in controlled environments, and those for desktops tended to top out at about 30 MB in the late '80s, or about 8 GB by the turn of the millennium. Those were also prone to head crashes if power was interrupted, and even by the user turning off his or her computer without "gracefully" shutting it down first, or restarting it too quickly after shutdown. The heads are designed to avoid that sort of crash now.

I run a small computer shop and I have seen a fairly large number of failed hard drives (although, blessedly, never on any computer I have owned myself -- just lucky, I guess, but mine are all on UPSes, which helps). Most of the hard drives that have been brought to me have been on laptops, which tend to fail more frequently because of the physical shocks they are subjected to. The failures on desktop drives are less commonplace, and seem to occur largely as a result of power failures, but lately I have seen a few from what I suspect to be a different cause.

Some newer drives do not spin at a constant rate, like all the old ones did, but vary the rate according to computed load factors and whatnot. I have seen failures on more of these than on the constant spin rate types even though they have been around for a shorter time, and I believe it to be the fault of the circuitry that accelerates or decelerates the rotational rate and the varying voltage load it puts on the little motor that makes the disks spin.

The price varies so greatly because of many factors, one of which is the robustness of build as reflected by the MTBF. Greater differences, however, include the fact that the faster disks spin, the more expensive the drive tends to be. We now see three common spin rates on consumer hard drives, 5,400 rpm, 7,200 rpm, and 10,000 rpm. This affects data throughput speeds, although from the standpoint of actually noticing the difference on your PC it is not by as much as you might presume, with faster speeds providing nothing like the improvement in performance that you get by going to an SSD (which you DO notice, and BIG TIME!).

One interesting aspect to the price is that, over the course of recent months, they skyrocketed for a while. This was due to supply/demand issues caused by floods destroying major disk drive factories in Thailand. They have now come back to earth as equilibrium was restored to the market and, as they almost always are no matter what "today" is, they are again cheaper today than ever before.

What I enjoy is the recent abrupt decline in prices for SSDs. I don't recall ever seeing the prices on anything fall so far so rapidly except maybe for specific models of smartphones when bought with a 2-year contract (Droid RAZR Maxx for FREE now???). What was $350 six months ago for a given SSD is $150 today (and no two-year contract is required!).

But disk drives are moving parts, and they are therefore prone to failure. Always have been, always will be. SSDs, which AREN'T moving parts, are reputed to have reliability issues of their own, but I haven't seen any yet. It is ALWAYS a good idea to keep your data backed up no matter what you normally store it on.