Can anyone explain these prices for me?

There's a certain minimum cost to create a drive, covering the materials, labour, shipping, and a pile of other stuff. This doesn't really change based on the capacity; only the equipment needed to make it, at a guess. It doesn't cost much less to make anything smaller than ~500GB.

750GB drives seem to have gone out of fashion for some reason. 640GB even more so.
 
There are also different product ranges. For example a WD Caviar Green is substantially slower than a Scorpio Black, and tend to be much cheaper than other product ranges from the same manufacturer.

Then there are also marketing ploys - for example the same Caviar Greens used to come in SATA II (ezrs) or SATA III (ezrx) - even though you'd be lucky to see a transfer rate above 30MB/sec.

Also as the above user stated, it may just be collecting dust...
 
I'll do one tomorrow if you really want but I bought a load a few because I thought the benchmarks on hardware canuks looked pretty decent for storage drives and I remember being quite dissappointed with the actual product. It is possible that I'm recalling the wrong HDDs though, I have a few rigs. I might end up with a red face here hahaha I still think 80MB/s is generous though and I think that we still both agree that the SATA III update was unnecessary 😉

ATTO, CrystalDiskMark or something else - what would you like?