I think it's unlikely, highly unlikely, that $15 a tonne will achieve the level of emissions reductions we need to achieve in order to avoid a dangerous disturbance to the climate system. I'm not aware of any serious study that says that.
That said, the comments that have been made about competitiveness really do matter. The answer has to be contingent on what our big competitor to the south does. If I were the czar of climate policy, I would continuously push just a little bit ahead of where the U.S. pushes.
In the end, we're going to need costs more like $50-a-tonne CO2, but I wouldn't just impose them instantly, because then there would be hideous implications for competitiveness. You do have to tie it to what other major competitors are doing.