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.
Given a device that hashes at 50GH/s for $2499 (
https://products.butterflylabs.com/)
The first day at 80,000,000 difficulty, the device generates 0.314321 BTC.
Here is the series for bitcoins generated per week for 28 weeks with a 9.4% difficulty adjustment per week starting at 80,000,000 difficulty.
2.2 + 1.99 + 1.81 + 1.64 + 1.48 + 1.34 + 1.22 + 1.10 + 1.00 + .90 + 0.82 + 0.74 + 0.67 + 0.61 + 0.55 + 0.50 + 0.45 + 0.41 + 0.37 + 0.28 + 0.23 + 0.21 + 0.19 + 0.17 + 0.15 = 21.92 ($2126)
On the first day of the 28th week the device generates 0.0218 BTC
I checked that series with the following calculators and using a spreadsheet.
http://www.bitcoinx.com/profit/http://tpbitcalc.appspot.com/http://www.coinish.com/calc/#https://bitclockers.com/calc