I was just wondering, by what logic could a CF card be used as a CHEAP harddisk?

I mean... Sure, you can't buy new hard disks in sizes comparable to (=as low as) the sizes of the cheap compact flash cards, those are the HD sizes you get for free from anyone, if you just ask.
And when going upto the "lowest size new HD", the matching compact flash card (with an adaptor) is way more expensive.