Had to have a giggle this afternoon.
Spent a bit of time at Clark Rubber and Bunnings and, to take a new (say $500 compressor) like the one I have now and make an enclosure for it:
$200 - Sound deadening foam
$150 - Sheet of decent thick ply
$150 - Decent muffler and plumbing for intakes
$ 40 - 300mm ceiling fan
$100 - Replacement flex discharge hose and fittings
$ 50 - Rubber mounts for the compressor and enclosure
$ 20 - Weatherseal adhesive strip
$ 50 - Various hardware to hold it together
Or nearly $800 for a decent DIY sound box for a standard compressor - assuming it's a compact direct drive like the one I have now, a belt drive will need more foam and timber.
So I'm up to $1300, which makes the Sydney Tools ones start looking a little less painful...