I used this calculator with presets for a 3 module miner.
An additional step should be to discount each future week to the present value using some interest rate. I have not done that step.
This gives me a value of about 70 BTC for an Avalon.
I am sure that many people would use different numbers for this. For the power, I used 650 W * 80 GH / 65 GH --> 800 W, as it is overclocked.
I'd like to hear what other's think.
I think your model is too conservative.
Again.. who knows..
So, in 25 weeks, we will see 24x current difficulty? Quite surely, before that, many inbetween miners, that has been hosted on offices for free but finally only doing pennies will shut off. Along with other miners shutting off aswell.
I dont believe the increase can be that steep even if half the promised hardwares start shipping. (from 50,810,339 to 1,226,438,065 in 25 weeks (6 months!..) but anyway, we will live to see the proof for sure!
adding 70M per month will look something like this:
http://mining.thegenesisblock.com/a/fc54d24eaa
roi in 210 days on 50 BTC..