Green’s Dictionary of Slang

bet! excl.

also that’s a bet!
[you bet! under bet v.]

(US) a general excl. of affirmation or agreement.

[US]M. Baker Nam (1982) 41: I was given to Fox Trot [company]. I said, ‘That’s a bet. I like that name.’.
[US]Eble Campus Sl. Mar. 1: bet – affirmative answer. ‘Ready to go?’ ‘Bet!’.
[US]Da Bomb Summer Supplement 2: Bet (mt.) cool; all right.