tell you what

Ellipsis of I'll tell you what or I tell you what.

tell you what

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

• “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.