It costs $5 to setup an account (at least thats what it was when I did it) How much you spend is up to you. I have one campaign that I spend 2 or 3 dollars per month on.
I always wondered what lowest CPC was aswell - thought it was a silly question - so I am looking forward to an answer.
The $5 set up fee is applied to the cost of clicks if I remember correctly?? The minimum bid for keywords is relative to the keyword so it can vary greatly. Some can be as low as 1 cent but I think that this is very rare as I have never been able to bid 1 cent on anything ever.
I wonder what they use the $5 fee for? Infrastructure, personnel that checks campaigns and such. Just wondering...