informal, informal something that you say when you do not believe that what someone has just said will be true.
'I don't think she'd be stupid enough to lend him any money.' 'How much do you want to bet?'
How much do you want to bet?
I'm so confident that you are wrong that I'm willing to bet on it. A: "There's no way that's the capital of Texas. I would have remembered that." B: "How much do you want to bet?"