an answer here:

http://laurent.henocque.com/post/2013/09/16/On-Bitcoin-Mining-and-the-vallue-of-Bitcoin

at current mtgox price 139$, you should not pay now more than
16 dollars/GH if you expect mid october delivery
11 dollars/GH if you expect early november delivery
8 dollars/GH if you expect mid november delivery
6 dollars/GH if you expect early december delivery
4 dollars/GH if you expect mid december delivery
3 dollars/GH if you expect early january 2014 delivery

this is for raw break even. If you want to be on the safe side, these figures should be divided by four (one month delay, plus targetting to double your BTC)

or the value of a BTC should be multiplied by 4.