I've done this before, I bought from a company called X-case
the reason I did it was the company that I worked for at the time had grown from a garage startup, and had a lot of important machines that were in desktop cases, it meant in the server room there was more shelf racking needed to put stuff in than space really allowed, and a mostly empty 42U coms rack with about 5 1U switches in it...
If you're gutting a regular desktop, then you will need to allow for full size vertical cards to be used (if you have any) and for that you need a 4U case.
if you're not using any expansion cards, then you can obviously use a smaller case.
(even down to 1U - I've seen small ATX boards in as little as 1U cases before (and they do work well)
If you're gutting a desktop that use non standard mounting holes, (like Dell) or a riser and horizontally mounted expansion cards. then a standard ATX case won't go it for you.
for an idea of price:
they start on that site £35 for 1U £50 for 4U (you should not pay Tax on items exported from the UK)
£50 is $80 canadian... ($61 USD) (you're talking about buying through a friend in Canada I'm not sure where you are based)
$300 should get you a very good case, - that's a lot more than off the shelf prices, - but if you're talking about going to a custom fabricator, that's probably a fair price...