it will really depend on the floor dimensions of 40 gal breeder tanks. I really don't think anything less than 4'x2' of floor space and between 16"-24" of height is good for a single dragon and for 2-3 dragons, 5'x2' or variation (ie 3'x3', 4'x3' etc) or larger.
As you indicated you do not have a lot of money to spend on cages, you can build one for alot cheaper than most 40 gal glass tanks would cost.
Even a simple 'box' isn't too difficult to build, just a matter of measuring the sides, bottom, top and front/back pieces, taking in account thickness of the wood for the end pieces (ie a 4'x2'x2 cage end pieces will need to be 24"x24" plus width of the wood for each side..ie, if you use 1/2" thick plywood or melamine you need to add an additional half inch to each side of the end pieces, thus resulting in 25"x25" squares for the end pieces (24" + (.5" x2) (one of left side one on right side) = 25" for the length and same for the height. Thus the end pieces will fit againsat the sides top and bottom, if the base is considered the 'central connecting piece', ie the long sides attach to the end of the base, and the top sits inside the front/back pieces (or it too will need to be slightly wider, to fit ontop of the front/back pieces.
If it sounds all confusing, build a 'scale down' model of your cage in cardboard or foam core (basically bristle board with foam in between, many 'display' panels are made out of it). As cardboard is easy to come buy and cheap, using it works well.
The way I scale down feet into a more manageable test scale, is one foot equals 4", thus a 4'x2'x2' cage would be 16"x8"x8" and as cardboard is typically about a quarter inch...the thickness would mean 1/4" of board would mean 3/4" wood. Or don't worry so much about cardboard thickness translating into wood thickness...but you can see what pieces will need to be increased to fit properly to for a large rectangular box. Just add the thickness of the board to the pieces which will need to overlap other pieces.
The most trickiest part is designing the door/front of the cage. If you have skill saws, drills and all the nice power tools, it is much easier, you can just drill a hole big enough for a skill saw and cut a rectangular with a 2"-4" boarder all around to form your door frame, or one side of the 'sandwich' to mount plexiglass or sliding doors. A door or pair of doors which open from top to bottom, works good as you can open the cage, and keep the door open without worrying about it falling on your head...of course this means the cage has to be up off the floor, which is best anyway, so the door can swing out of the way. YOu can have it open upwards but then you need something to hook it on, so it doesn't fall on your head or needing to have one hand free to keep the door open while you do whatever in the cage. Trust me, for cleaning, or moving an adult dragon or two one handed, can be a pain, and not very comfortable for your dragon. Two hands is always best, this way one hand can help support and hang onto the dragon.
For gages longer than 4' I suggest splitting the door into two pieces, this way you can open only one side or the other and it swings to the left or right. Sliding glass/plexiglass doors can be made to do this, overlapping their ends in the middle, so there is no space for anything smaller than a cricket to get out (or not even that.)
The top can be solid or half hardware cloth, depending on how you plan to mount the light fixtures.
For examples of home made cages, check the enclosure photo gallery. Even if the cage is not designed for bearded dragons, it typically doesn't take much more conversion to make it perfect for dragons.
Another great point about designing and building your own cage, is you can make them stackable,so you can have two smaller cages on top of each other, this way, if your pair don't get along, or you want more, you can make use of vertical space.
If building your own cage is something you are not sure you can do, or wishing it looked nicer overall, converting discarded gently used furniture can be done. A long dresser, or a tall wide one, can be gutted (removal of drawers, runners, etc) and doors made on the existing frame, additions/improvements on the backboard and top to allow for fixtures and prevent escapes and ensure proper support of the floor (sand, rocks, basking spots, dragons can all together weight more than dressers may be designed to support, but adding thicker wood underneath can easily provide needed support etc). Wall units, lain on their sides, wooden headboards for beds that are equipped with wide/deep shelves, can also be converted. Those large floor model TV sets could also be made ,rip out all the 'tv' parts and you have a nice square display setup for your dragons (should be pretty large to ensure room for a single dragon, floor space approximately 8 square feet or more, for two dragons)
Big clunky coffee tables, end tables etc, can also be converted. Big tall wardrobes and china cabinets can be, again, tilted on their sides, or if wide enough, used set up as stacked cages, with room for two or more dragons in each section (or single dragons in each section).
Many times I see suitable funiture sitting on the curb night before garbage pickup near my place, especially where there are apartments. CHeck the local apartments, many landlords just toss out furniture that is left behind by tenants moving out. If they are dry (ie it didn't rain on them before you found them, if you know your garbage days etc in your area, then you will get an ideal how long the stuff is out, and you will know if it rained during the day or early evening). Course you will need access to a van or truck (and a friend or two) to help you collect the stuff.
Used stores often have old furniture that can be bought really cheap and converted for not much more additional cost.
Oh, one piece of advice, if you do find the ideal piece of furniture out on a curb, if it's not really late at night, it is a good curtesy to ask the owner of the curb (ie if its infront of a house, ask whoever owns or lives in the house, if an apartment, buzz the super or landlord) if they want 'insert name of furniture' on the curb and if they don't, can you take it? Most will just say go ahead. Be kind and don't spread whatever is piled ontop of it all over the curb/yard/road, if you need to 'unbury' your future cage, pile things back up neatly after you load it into your truck. This way, when you are looking for a new piece of furniture and see on at the same place, they won't say no. Nobody wants to look out after you asked to take something and find all their garbage spread all over their yard. If you do and the guy is a real vengeful sort he may just call the cops say you stole the furniture from his yard, probably have a good description of your truck etc....and that wouldn't be very good.
Well this post turned out from a simple suggestion to a near book on 'how to build your own cage and where to get the materials'. I didn't intend it to be this long..but..well, it all just flowed out.
Just for the heck of it, below is my homemade cage (built from scratch by a friend of the family...based on my drawings. Though now, I would have changed a few things...for one thing, insisted on proper corners (ie like picture frames, with the 45 degree cuts so the corners are nice) and I wouldn't have included the mini door at the bottom. I put that in as i was thinking of using washed play sand ,thus i could more easily sweep it out of the cage, without having to lift it over a ledge. By the time the cage was built, I changed my mind on using sand at all...realized it would take a lot of sand to give an inch or so layer for the entire cage...decided it was just too much mess to deal with not to mention buying and lifting 50 pound bags of sand every couple of months. The cage is 5'x2'x2'
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PHLdyPayne
