When I bought my cage a couple months ago, I purchased it at a farm store
(called Central Tractor).  The cage I bought was 24x24x16 and it cost me
$16.00 (yes sixteen dollars!).
 
They had larger ones that were 30x30x24 or something, I just cannot remember
now, and those cost $25.00.  (note: these cages do not come with the
drop-pan, that needs to be purchased seperately)
 
The only catch is that you have to assemble these cages yourself, all you
need is a pair of pliers, because they includes the "clips" and all the cage
walls/door, etc... There is a certain type of pliers that are made
specifically for fastening those little "clips" around the wires of the cage
to keep the sides together.  But i just used regular pliers.  It did take a
couple hours to get the cage completely assembled because the pliers were
strange to work with at first, and because you have to fasten the "clips"
every 2 inches or so.
 
But now I have a very sturdy cage that seems to do the job just as well as a
pre-assembled one.  My ferret is loose all day so there is no question of
him being in a too small cage.  And I only put him in his cage when I go to
bed, and he is good enough to sleep through the entire night with no
problems.
 
Hope this helps!!
Aprille
 
[P.S.]
I forgot to mention that It would be very easy to stack these cages up or
assemble them any way you like, inorder to have a much larger cage with 2-3
(or more) levels.
[Posted in FML issue 1709]