A friend and I, Chris (chris_harper2), built a cage out of oak laminated plywood that is 4'x 3' x 30" (LHW)... We used those demensions to maximize the amount of wood we had at our disposal with one sheet of plywood, to go any bigger, we would have been cutting into two sheets... Granted your cage will only be 24" wide, you will fit through most standard doors no prob so that's ok... Keep in mind that a cage of your demensions is going to get pretty heavy when its all said and done...
We did everything the right way, and took no short cuts... The thing is a piece of furnature and a living room display cage so there are some things that a person could do to make it cheaper...
We used very simple tools, but there were MANY man hours put into it... It took us about 3 months to finish, mostly because we are very busy people, but partially because there were steps that we took that we simply could not rush (ie wood finish drying times and various other things)...
We didn't use any sort of frame so to speak, we simply screwed the pieces of plywood together using 90 deg. corner braces... Chris may have better ideas, but I would say you do not need a 2" x 4" frame and you could build the same way we did...
After buying the plywood, the face treatment molding, the wood for the screen frame, the screen, the stain and polyruethane, the epoxy to treat the inside of the cage ($100 dollars for that alone), the glass track and 1/4 inch glass ($75 dollars here), all materials (ie sandpaper, screws, ect) and the lighting and plumbing supplies, I spent about $500 I would say... It certainly wasn't less than that... BUT, this was more for entertainment than neccesity, as both Chris and I love building things of this sort...
Depending on the amount of detail you want to put into your cage, it may be best to go with a pre made design... If not for cost, then for the advantages of being able to break it down to move or clean... Chris, can you post that picture of our cage if you have it on file?
-----
Ben