I thought of building a cage out of a cabinet as well. I didn't like the lack of ventilation and while I'm sure people with powerful enough ACs and such can manage, I knew my little room AC would probably not be able to compensate for the trapped heat and humidity in the cage. Plus, when I move back home I am going to pay the electric bill for the AC so it would save me money if I can cool the cage more efficiently. And it took me a while to find cabinets that fit the size I needed them to.
I ended up building my own cage out of KD pine; melamine base and back wall; 16 gauge, galvanized after weld, 1"x1/2" wire I found at Lowe's; and poplar edging to cover the melamine edges and edges where the wire is stapled to the frame (which I want to replace with metal now I just don't know how to go about doing that). I have an album chronically the building of the cage in my profile.
If you don't have any tools, it will likely be cheaper to buy a cage. If you plan on building more than one than it might be worth it to buy the tools. Of course, if you have the tools (drill, wire cutter, staple gun, screwdriver, hammer, saw if you cut your own wood) and some spare hardware (braces, screws, hinges, door locks) and all you need is materials, you can build a large cage for a decent price.
Just think it through and plan everything. I drew up schematics and added up costs to find totals for everything. If you have a cage that is decent now, you can take your time and build one panel at a time.