Wow.. there is so many ways of seeing an evaluation of such machine.
1, why are you looking to buy?
a, To make your coins back
b, to make your coins back and start making profit
2, do you belive the bitcoin currency will:
a: increase in value
If you believe the coins will increase in value, against the usd, you will make money as long as it mines currently more than 1 coin per month (it costs you roughly $100/m on 800-1000watt OC on 3 boards)
b: decrease in value
If you think the btc will decrease its value due to fact of more sellers than buyers, since people will need their money back asap on their investments, what else can you invest with the new mined btc? do you believe its better to mine the coins than to purchase them?
3, do you think the machine has a resell value after you mined back say 75% of your investment? (and when have you mined back 75%, can you afford to do it in 8 months?)
4, Some investments has up to ten years to make full roi, how long are you willing to wait to make all your money back? 1 year? 3-6 months?
5, the current huge rises in difficulty, do you really believe them to be maintained?
a, yes. Then i think it could be difficult to make the coins back within 12 months
b, no. look at this site
http://mining.thegenesisblock.com/, on difficulty increase, instead of using percentage. put MM and play with past numbers instead of percentages, say 20-40 per month, or even 60. But would it make sense with a continued 90% per month increase ongoing?
I currently valuate batch 2 & 3 , 3 boards at 65btc. you will maybe make back 50-70% of that in the first 3 months pending on difficulty rise. Some sellers are not selling them lower than 99 btc STILL!
Question is then, when you made back your 50-70%, would it be worth 32.5-45.5BTC? i believe yes, then you lost nothing and you can get your coins back at lest. What would BTC/USD be evaluated at that stage. who knows.
The price will drop exponentially on each difficulty increase, if there are enough sellers who want to get out rather than continuing getting less and less mining.