If its $3 a day, that's only for 10 days, then diff changes again and now its $2 a day, then another 10 days and its $1 a day.
Nah, not quite that fast. It depends on how fast they get those BFL chips hashing on the DIY boards, but it probably won't drop below a buck (with current USD/BTC0 until late October or early November.
CAVEAT: I think there will be 3 PH/s by that time. There's just too much hardware out there slated for delivery; I don't think it can
all be late.