$1.5 per GH/s is stretching it, but possible with cheap electric or hosting
$2 per GH/s and up, FUGGEDABOUTIT
So basically only antminers are reasonable for now )?
I'd shoot for Antminer S2's at the right price point, because Bitmain revealed they'll provide inexpensive upgrade kits in a couple months to OC your 1 TH/s miner to close to 2 TH/s.
That or look out for a good Spondoolies Tech Groupbuy for the SP10 or SP30.
Also, look out for Friedcat's Asicminer based chips with any decent hash and power efficient miners. R-Box sucks balls, the Avalon Gen 3 800 GH/s sucks also, but maybe another integrator will come out with something sorta worth it.