Depends where you are. In Cambridge, Chivers sell used Juice concentrate containers for a very reasonable fiver or so, or at least have in the past - but I'm not sure they are even on that site anymore.
This lot:
http://www.deltacontainers.com seem reasonably priced, but delivery is a bit steep if you are nowhere near manchester
I've seen other suppliers of used large kegs fairly cheap, but can't find any now. This is the best I can find:
http://www.waterbuttsnbottles.co.uk/kegs.htm (new, so still 30 quid for a 220l keg) and
http://www.water-tanks.net/ who have all sorts of quite big tanks, but nothing that I would call cheap.
Someone agricultural on here might know the right place to go. 40 quid for a water butt does seem to be taking the weasel pee, which seems to be the standard garden centre rate. At a fiver each we got 5.