Getting hard to find a reasonable size case in aluminum for reasonable bucks. Might check out the Ultra m998 (not too small, but is reasonably priced - has a stainless steel mobo drawer so it's not particularly light either) - they were supposed to have some other aluminum boxes coming along, but no sign of 'em.... The Lancool series by Lian Li has aluminum skin on steel frames. Maybe you could find an Antec p160 out there somewhere. Other than that you have Lian Li and Silverstone for aluminum boxes and their prices will generally drain your bank account, but there are a few in the LL line that can be had for under $100. + sh.
.bh.