BFL ASICs ROI gets shaky with a $60 bitcoin at 1,500,000,000 difficulty ... but I guess there's Europeans with 50c per kilowatt/hour power bills.
I think you added a few too many zeros.
http://www.coinish.com/calc/#BFL 50GH/s @ $2499 (straight from their product page) will never generate a return @ 80,000,000 difficulty, $94/BTC and 1.2788% growth rate.
ASICMiner is looking at this same curve. Very soon it will not make sense for ASICMiner to deploy their own hardware.
ACTUALLY:
At 640,000,000 difficulty a BFL SC SINGLE will still be making ~ 1.32 btc per month. You're numbers are just plain wrong and ASICMINER about twice as power hungry as bfl gear (and 2/3 as avalon).
~
If you had a SC SINGLE in hand right now - you'd roi in under 2 weeks. @ 80,000,000 difficulty it's still under 3 weeks.