the antec site lists both the gamer 900 and the p180 as 31 pounds. i can tell you that the p180 is MUCH heavier than the 900.
there is some weight in the 900, but it's nothing to write home about.
i can't vouch for weight on the cosmos cases as i've never lifted one of either the 1000 or the S series'.

i found the 900 just as heavy as my p180 and p182 cases but the p180 and 182 are larger cases too.

I've done both a Cosmos 1000 & a Cosmos S build in the last month so I've got the straight gen on weight fully kitted.

Cosmos 1000 with stock cooling, a PC P&C 750W silencer, and an ASUS 3870X2 = 60lb

Cosmos S with 320 rad, Corsair 750W, and a HIS ICEQ3 3870 = 50lb


Looking at the pics of the antec 1200, it looks to be as tall as the Cosmos S (judging by the bottom mount PSU and the rooom up top for a rad) so unless it's made out of almost all aluminum it will probably weigh in at around the same as the Cosmos 1000 which lists as 17+ Kilos.

I'm not trying to talk anybody out of the 1200, that's a personal choice, but I doubt it will be any lighter than the Cosmos S. (Has a biggass side fan for cooling rather than the one on top of the 1200).

Exactly but I'd still bet money the 1200 will be lighter regardless. I weighed my 900 loaded & it was less than what Antecs site said for shipping weight bare so what does that tell ya? lol Don't believe the hype haha.