From Longman Dictionary of Contemporary EnglishI/I’ll tell you whatI/I’ll tell you what (also tell you what) a) SUGGESTused when you are suggesting or offering something I tell you what – let’s have a picnic in the park. b) American English used in order to emphasize what you are really saying I tell you what, I’m not looking forward to standing up in court tomorrow. → tell
Examples from the Corpus
I/I’ll tell you what• I tell you what, I'll make you dinner if you drive me to the store. 