Well - they could be a rarity because e-Bay UK doesn't sow any for sale, just RAM, a BIOS chip and a PSU. That said, I doubt one would fetch above £100 without a monitor in Britain because the market here for second hand PCs is dulled by the preference for laptops and more recently, tablets.
If you're letting go of the hard disk, don't forget to delete your personal files beyond retrieval. Spybot can make 35 passes at deletion and not much survives that - unless you're the CIA!
I can't speak for Canada of course although I suppose I could ask my daughter in Regina, Saskatchewan for an opinion. My price estimate applies only to the UK but I suspect the move to laptops, pads and tablets is worldwide. Aim for two hundred Canadian and hope to get near it.