tell you what

Ellipsis of I'll tell you what or I tell you what. The sense introducing an arrangement also corresponds to another, longer established collocation of I'll tell you what I'm gonna do or let me tell you what I'm gonna do.

tell you what

1. (colloquial) Introduces a compromise or arrangement where the interlocutor has some benefit or advantage.

   Tell you what, you can have it for fifty quid if you'll promise to give it a good home.

2. (colloquial) Synonym of I can tell you.

   Tell you what, you won't find another one that cheap anytime soon.

• “tell you what”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.
• “I tell you what”, in Cambridge English Dictionary, Cambridge, Cambridgeshire: Cambridge University Press, 1999–present.
• “tell you what” in Longman Dictionary of Contemporary English, Longman.