The Aspire X-Sonic is small and light weight - pretty much the same chassis as the original Lan Boy. They seem pretty flimsy out of the box (some would wonder if there is even a case in the box when they receive it, if it wasn't for the included, junky PSU

), but stiffen up a bit when you get your components in and tied down. Some Sky Hawk/Eagle (aka Skyhawk) cases (some places that still carry some: Newegg, mWave, Dealsonic) are a bit stronger - one of their MSR series I had here weighed about 12 pounds net and all the better ones left that I've seen out there lately use 1.0mm Al, while the MSR series was 1.2 mm, so they should be even lighter. I'd be wary of any Al made with thinner than 1.0mm Al as even 1mm can seem flimsy. These could be considered your budget options.
Aspire and Chenming have Al versions of some of their cases. And Enermax has some Al cases that aren't very expensive - I have no direct experience of the sturdiness or lack thereof of the Enermax. And directron has some remaining TTGI/Superflower Al cases they sell both rebadged as Directron and the original brand. These are your mid-range options. Directron has a long list of "Al" cases but many just have Al bezels, the rest of the case is steel (e.g. the CM Centurion series and Mystique).
Then there are Thermaltake, Coolermaster, Silverstone and Lian Li whose all-Al cases are all approximately as well made (some would differ on the Tts

) and as expensive. But most won't be considered too lightweight as they use pretty heavy gauge Al and have features like mobo drawers and heavy extrusions on the doors, bezel, etc. If your mobo is microATX as someone mentioned, take a look at the new Silverstone SST-TJ08. It's not really light and is quite expensive for a microATX case. But it is nize. Aopen has one Al case in current production, the Nouveau - you can find pix of it on Newegg. It comes with a decent PSU
. Lian Li probaly has the edge due to their endless number of models and variants, some of which are pretty pared down on fearures so they should be lighter than their higher-end boxes. But I just don't like L-L the company - they seem arrogant to me and their cases are often lacking in features that are expected even of $50. and below cases these days.
And there is one AOpen
A600A (discontinued model) available on mWave.com that would normally be considered in the high-end category but selling in the mid-range price area. Unfortunately it comes with a PSU that you might not be able to use as it is probably an old design with a relatively weak 12V rail, but it is still a good Fortron PSU that should bring some money on eBay or in the FS/T section here. I have no idea as to the weight - looked it up - net wt. is about 15 pounds (but IDK whether that includes the PSU).
.bh.