What do you want to bet that you don't know what you are talking about? I'd bet even money that you don't know what you are talking about! When you think you hear 'want a bet' you are really hearing 'wanna bet' = want to bet. If you want to go even more colloquial, you can say 'you are talking s***, you know that?' (that means, it's nonsense what they are saying, or it is wrong)