Are we talking survival here, or do you want it to keep its foliage or would it actually be growing?
I would say keep it above 45F if you want foliage to survive. Above 65Fif you want it to actually grow. Below about 35F really bad things begin to happen involving dieback, which I can personally vouch for.
There are more variables to this, including wind and dehydration.
Regards,
Matt
A ficus link:
http://www.missico.com/personal/tid...mina_bonsai.htm