I couldn't care less (BE)
I could care less (AmE)
I don't/couldn't give a monkey's. (This is very informal, and potentially offensive)
Whatever. (Originally AmE I think, but growing in popularity over here.)
My indifference knows no bounds. (I might have just made this up, but I can imagine saying it. )
So what's new? (said after someone's given unsurprising 'news')
And? (the same)
PS - Some US participants in this forum disapprove of "I could care less". But it's very common, and definitely not BE.