Person B: *yawn* Oh really? I couldn't care less.
But it can also be interpreted to mean "I care about this to a certain extent (which may be a little or a whole lot) and there is no way in hell my caring is ever going to drop."
Arguments about this can get pretty heated, with Team Couldn't Care Less tending to care more.
"Please don't bother me with that, I really couldn't care less about it."
"Mr. Politician, do you care about America?"
"I'm glad you asked that. I care about America more than anything in the world, and I couldn't care less, not in a million years. You'll have to pry my care from my cold dead hands."