A 12 foot diameter inflatable rim pool with filter for $49 NOW $39 (normally these are over $100). I saw these pools at K-Mart North Rocks (Sydney) but usually when they clear them at one store, it's national.
EDIT TO ADD : The 10ft is $29 and the steel frame 12ft is $79
On eBay the 12ft are currently $89 + Freight - see http://tinyurl.com/7oee8y6
ShoppingSquare used to sell them for $119.95 + Freight http://tinyurl.com/6uthkaq
The 10 foot pool got down to $69 on special around Christmas (normally $99), but the 10ft is not currently on clearance in my local K-Mart store. Last year they eventually dropped the 10ft down to $30 to get rid of them, might be worth keeping an eye out for those too.
NOTE - these pools must be placed on a perfectly horizontal surface, so that the water depth around the entire edge is equal. If not, they tend to flop over to one side when kids lean against the upper edge, since there is more water weight on that side (make sense?)
The easiest way to level things is to use brickies sand (Bunnings) and a spirit level to make a nice circular base. It can be deeper in the centre, but you need it to be pretty much flat around the edges.
Cheap pool. But the council permits, fencing and inspections will not be so cheap.
At least these are easy to clean.