If you really want to save money, you have some time, a few tools with a Dremel drill and/or Radio-Shack nibbler-tool, you might want to engage in my own ongoing obsession of recycling used full-tower cases from vintage (ATX compatible) OEM computers.
My preference is Gateway or Gateway 2000 cases, but I have obtained at least one InWin full-tower that was probably made and marketed around 1999.
You shouldn't need to look hard to find FREE cases. (I see recommendations above for cases priced at $29 or so, but consider (a) the joy of planning the project, (b) the joy and skinned knuckles of doing the work, and (c) a FREE computer case.) In most major cities, you can find "Computer Surplus" centers, where a small company has rented warehouse space to receive cyber-junk and recycle it. They make their money as much from recycling the precious metals and computer-case sheet metal, and I have found -- INVARIABLY in both Southern California and Tacoma, Washington, that all you need to do is collect some of your useless cyber-junk, dead hard disks, burned-out motherboards, and obsolete phone modems or network cards, then visit one of these warehouses, where they will gladly take your junk and allow you to select a bare computer case of your own choosing.
If you need additional sheet-metal panels, the stuff can be had for a few dollars in 2'x3' sheets at a local sheet-metal store, and they also sell the equivalent of "modder's mesh" or perforated sheet-metal. Beyond that, you only need to obtain a cheap $5 or $10 pop-rivet tool at an auto-parts store, and you should be ready for about anything.
Some people prefer Aluminum computer cases, like the Lian-Li models described in this thread. I AM very much impressed with the Lian Li V-1x00 and -2x00 cases, and they manufacture some marvelous full tower cases. I also like the CM-Stacker cases. But what I like even more is not spending $200 or $300 for a computer case.
It is also true that aluminum conducts heat three times more efficiently than SECC steel sheet-metal. However, if you design your computer system with cooling efficiency in mind, the amount of heat-reduction achieved by an aluminum case will be insignificant. I can also say that the use of sound-deadening pads, while it also reduces the cooling capabilities of either aluminum or steel, does not have that much of an adverse effect if you strategically choose your fan-deployments, ducting and other techniques -- especially toward the goal of sequestering warmed-up air and getting it out of the case before it mixes with other air inside the case.
How does "FREE CASE" sound to you?